Condvar: Don't context switch between two unlocks

Before waiting, condvar_wait releases two locks - the user's lock and its
internal lock. If we reschedule after the first unlock, a waiting thread
may start running, and hang when it also does condvar_wait. So let's use
preempt_disable/enable around the two locks.

This patch improves single-CPU performance of the cond-perf benchmark by
about 10%, but this is an extreme case (cond-perf tries to do condvar_wait
almost immediately after waking up from its previous wait).