callouts: perform wake() outside of lock.

given the scheduler state, wake() sometimes rescheduled the dispatcher thread
immidiately, and then it blocked on the mutex that is still held by the caller
of _callout_stop_safe_locked().

this patch does wake() outside of the lock to eliminated these spurious context
switches.

bsd/porting/callout.cc

@@ -69,10 +69,14 @@ namespace callouts {
         return (false);
     }
 
+    void mark_have_work(void)
+    {
+        _have_work = true;
+    }
+
     // wakes the dispatcher
     void wake_dispatcher(void)
     {
-        _have_work = true;
         _callout_dispatcher->wake();
     }
 }
@@ -225,9 +229,11 @@ int callout_reset_on(struct callout *c, u64 to_ticks, void (*fn)(void *),
     c->c_flags |= (CALLOUT_PENDING | CALLOUT_ACTIVE);
 
     callouts::add_callout(c);
-    callouts::wake_dispatcher();
+    callouts::mark_have_work();
     callouts::unlock();
 
+    callouts::wake_dispatcher();
+
     return result;
 }
 
@@ -248,6 +254,7 @@ int _callout_stop_safe_locked(struct callout *c, int is_drain)
 
         // Wait for callout
         callout_set_waiter(c, sched::thread::current());
+        callouts::mark_have_work();
         callouts::wake_dispatcher();
 
         sched::thread::wait_until(callouts::_callout_mutex, [&] {