This will be pushed separately as MDEV-24142 once it works.

rw_lock_sx_unlock_func(): Wake up an X-latch waiter if needed.

sync_array_wait_event(): Wake up even if a pending SX-latch
request exists.

rw_lock_x_unlock_func(): Clean up.

FIXME: rw_lock_x_lock_wait_func() can still get stuck in
sync_array_wait_event() with lock_word==-X_LOCK_HALF_DECR.