Make sure that spin_unlock_wait() is properly ordered wrt atomic_inc().

(akpm: can't we convert this code to use rwlocks?)
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>