Fix WakeupWaiters() to not wake up an exclusive locker unnecessarily.

WakeupWaiters() is supposed to wake up all LW_WAIT_UNTIL_FREE waiters of
the slot, but the loop incorrectly also woke up the first LW_EXCLUSIVE
waiter, if there was no LW_WAIT_UNTIL_FREE waiters in the queue.

Noted by Andres Freund. This code is new in 9.4, so no backpatching.