slow_work_thread() should do the exclusive wait

slow_work_thread() sleeps on slow_work_thread_wq without WQ_FLAG_EXCLUSIVE,
this means that slow_work_enqueue()->__wake_up(nr_exclusive => 1) wakes up all
kslowd threads.  This is not what we want, so we change slow_work_thread() to
use prepare_to_wait_exclusive() instead.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/slow-work.c

@@ -372,8 +372,8 @@ static int slow_work_thread(void *_data)
 		vsmax *= atomic_read(&slow_work_thread_count);
 		vsmax /= 100;
 
-		prepare_to_wait(&slow_work_thread_wq, &wait,
-				TASK_INTERRUPTIBLE);
+		prepare_to_wait_exclusive(&slow_work_thread_wq, &wait,
+					  TASK_INTERRUPTIBLE);
 		if (!freezing(current) &&
 		    !slow_work_threads_should_exit &&
 		    !slow_work_available(vsmax) &&