shmem: don't write zero pages into an image
shmem: don't write zero pages into an image

https://github.com/xemul/criu/issues/260
https://travis-ci.org/avagin/criu/builds/182462796
https://ci.openvz.org/job/CRIU/job/criu-patchwork/941//console
v2: don't call page_pipe_add_hole for zero pages
Signed-off-by: Andrei Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu