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>