six locks: Fix a lost wakeup

six locks: Fix a lost wakeup

There was a lost wakeup between a read unlock in percpu mode and a write
lock. The unlock path unlocks, then executes a barrier, then checks for
waiters; correspondingly, the lock side should set the wait bit and
execute a barrier, then attempt to take the lock.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>