sched/__wake_up_sync_key(): Fix nr_exclusive tasks which lead to WF_SYNC clearing

Only one task can replace the waker.
Signed-off-by: NKirill Tkhai <tkhai@yandex.ru>
CC: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/512421372963700@web25f.yandex.ruSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -2660,7 +2660,7 @@ void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
 	if (unlikely(!q))
 		return;
 
-	if (unlikely(!nr_exclusive))
+	if (unlikely(nr_exclusive != 1))
 		wake_flags = 0;
 
 	spin_lock_irqsave(&q->lock, flags);