sched: Avoid IPIs in thread::wake()

Avoid sending an IPI to a CPU that's already being woken up by another
IPI.  This reduces IPIs by 17% for a cassandra-stress run. Execution
time is obviously unaffected because execution is bound by lock
contention.

Before:

[penberg@localhost ~]$ sudo perf kvm stat -e kvm:* -p `pidof qemu-system-x86_64`
^C
 Performance counter stats for process id '610':

         6,909,333 kvm:kvm_entry
                 0 kvm:kvm_hypercall
                 0 kvm:kvm_hv_hypercall
         1,035,125 kvm:kvm_pio
                 0 kvm:kvm_cpuid
         5,149,393 kvm:kvm_apic
         6,909,369 kvm:kvm_exit
         2,108,440 kvm:kvm_inj_virq
                 0 kvm:kvm_inj_exception
               982 kvm:kvm_page_fault
         2,783,005 kvm:kvm_msr
                 0 kvm:kvm_cr
             7,354 kvm:kvm_pic_set_irq
         2,366,388 kvm:kvm_apic_ipi
         2,468,569 kvm:kvm_apic_accept_irq
         2,067,044 kvm:kvm_eoi
         1,982,000 kvm:kvm_pv_eoi
                 0 kvm:kvm_nested_vmrun
                 0 kvm:kvm_nested_intercepts
                 0 kvm:kvm_nested_vmexit
                 0 kvm:kvm_nested_vmexit_inject
                 0 kvm:kvm_nested_intr_vmexit
                 0 kvm:kvm_invlpga
                 0 kvm:kvm_skinit
             3,677 kvm:kvm_emulate_insn
                 0 kvm:vcpu_match_mmio
                 0 kvm:kvm_update_master_clock
                 0 kvm:kvm_track_tsc
             7,354 kvm:kvm_userspace_exit
             7,354 kvm:kvm_set_irq
             7,354 kvm:kvm_ioapic_set_irq
               674 kvm:kvm_msi_set_irq
                 0 kvm:kvm_ack_irq
                 0 kvm:kvm_mmio
           609,915 kvm:kvm_fpu
                 0 kvm:kvm_age_page
                 0 kvm:kvm_try_async_get_page
                 0 kvm:kvm_async_pf_doublefault
                 0 kvm:kvm_async_pf_not_present
                 0 kvm:kvm_async_pf_ready
                 0 kvm:kvm_async_pf_completed

      81.180469772 seconds time elapsed

After:

[penberg@localhost ~]$ sudo perf kvm stat -e kvm:* -p `pidof qemu-system-x86_64`
^C
 Performance counter stats for process id '30824':

         6,411,175 kvm:kvm_entry                                                [100.00%]
                 0 kvm:kvm_hypercall                                            [100.00%]
                 0 kvm:kvm_hv_hypercall                                         [100.00%]
           992,454 kvm:kvm_pio                                                  [100.00%]
                 0 kvm:kvm_cpuid                                                [100.00%]
         4,300,001 kvm:kvm_apic                                                 [100.00%]
         6,411,133 kvm:kvm_exit                                                 [100.00%]
         2,055,189 kvm:kvm_inj_virq                                             [100.00%]
                 0 kvm:kvm_inj_exception                                        [100.00%]
             9,760 kvm:kvm_page_fault                                           [100.00%]
         2,356,260 kvm:kvm_msr                                                  [100.00%]
                 0 kvm:kvm_cr                                                   [100.00%]
             3,354 kvm:kvm_pic_set_irq                                          [100.00%]
         1,943,731 kvm:kvm_apic_ipi                                             [100.00%]
         2,047,024 kvm:kvm_apic_accept_irq                                      [100.00%]
         2,019,044 kvm:kvm_eoi                                                  [100.00%]
         1,949,821 kvm:kvm_pv_eoi                                               [100.00%]
                 0 kvm:kvm_nested_vmrun                                         [100.00%]
                 0 kvm:kvm_nested_intercepts                                    [100.00%]
                 0 kvm:kvm_nested_vmexit                                        [100.00%]
                 0 kvm:kvm_nested_vmexit_inject                                 [100.00%]
                 0 kvm:kvm_nested_intr_vmexit                                   [100.00%]
                 0 kvm:kvm_invlpga                                              [100.00%]
                 0 kvm:kvm_skinit                                               [100.00%]
             1,677 kvm:kvm_emulate_insn                                         [100.00%]
                 0 kvm:vcpu_match_mmio                                          [100.00%]
                 0 kvm:kvm_update_master_clock                                  [100.00%]
                 0 kvm:kvm_track_tsc                                            [100.00%]
             3,354 kvm:kvm_userspace_exit                                       [100.00%]
             3,354 kvm:kvm_set_irq                                              [100.00%]
             3,354 kvm:kvm_ioapic_set_irq                                       [100.00%]
               927 kvm:kvm_msi_set_irq                                          [100.00%]
                 0 kvm:kvm_ack_irq                                              [100.00%]
                 0 kvm:kvm_mmio                                                 [100.00%]
           620,278 kvm:kvm_fpu                                                  [100.00%]
                 0 kvm:kvm_age_page                                             [100.00%]
                 0 kvm:kvm_try_async_get_page                                   [100.00%]
                 0 kvm:kvm_async_pf_doublefault                                 [100.00%]
                 0 kvm:kvm_async_pf_not_present                                 [100.00%]
                 0 kvm:kvm_async_pf_ready                                       [100.00%]
                 0 kvm:kvm_async_pf_completed

      79.947992238 seconds time elapsed