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 <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/shmem.c

@@ -711,8 +711,10 @@ static int dump_one_shmem(struct shmem_info *si)
 again:
 		if (xfer.parent && page_in_parent(pgstate == PST_DIRTY))
 			ret = page_pipe_add_hole(pp, pgaddr);
-		else
+		else if (pgstate != PST_ZERO)
 			ret = page_pipe_add_page(pp, pgaddr);
+		else
+			ret = 0;
 
 		if (ret == -EAGAIN) {
 			ret = dump_pages(pp, &xfer, addr);