dm bufio: use waitqueue_active in __free_buffer_wake

Save one spinlock by using waitqueue_active. We hold the bufio lock at
this place, so no one can add entries to the waitqueue at this point.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-bufio.c

@@ -1665,7 +1665,12 @@ static void __free_buffer_wake(struct dm_buffer *b)
 		c->need_reserved_buffers--;
 	}
 
-	wake_up(&c->free_buffer_wait);
+	/*
+	 * We hold the bufio lock here, so no one can add entries to the
+	 * wait queue anyway.
+	 */
+	if (unlikely(waitqueue_active(&c->free_buffer_wait)))
+		wake_up(&c->free_buffer_wait);
 }
 
 static enum evict_result cleaned(struct dm_buffer *b, void *context)