If shm->delta = MAX_DELTA and we set shm->stop to 1, the child can exit
before the parent adds MAX_DELTA to shm->delta.

parent			child
shm->stop = 1
			if (shm->stop == 1 && shm->delta == MAX_DELTA)
				break
shm->delta += MAX_DELTA

Cc: Konstantin Neumoin <kneumoin@parallels.com>
Reported-by: Konstantin Neumoin <kneumoin@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>