futex: Fix (possible) missed wakeup

We must not rely on wake_q_add() to delay the wakeup; in particular
commit:

  1d0dcb3a ("futex: Implement lockless wakeups")

moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which
could result in futex_wait() waking before observing ->lock_ptr ==
NULL and going back to sleep again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1d0dcb3a ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>