kfifo: fix kfifo_out_locked race bug

Fix a wrong optimization in include/linux/kfifo.h which could cause a race
in kfifo_out_locked.

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Reported-by: Johan Hovold <jhovold@gmail.com>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kfifo.h

@@ -228,13 +228,6 @@ static inline __must_check unsigned int kfifo_out_locked(struct kfifo *fifo,
 
 	ret = kfifo_out(fifo, to, n);
 
-	/*
-	 * optimization: if the FIFO is empty, set the indices to 0
-	 * so we don't wrap the next time
-	 */
-	if (kfifo_is_empty(fifo))
-		kfifo_reset(fifo);
-
 	spin_unlock_irqrestore(lock, flags);
 
 	return ret;