shmem: skip zero pages

page_in_parent() returns true for all not dirty pages,
probably it should not be executed for zero pages.

------------------------ grep Error ------------------------
(00.115327) Error (criu/pagemap.c:191): Missing 8000 in parent pagemap, current iov: base=9000,len=32768
(00.115329) Error (criu/page-xfer.c:237): Hole 0x8000/4096 not found in parent
(00.160222) Error (criu/cr-dump.c:1636): Dumping FAILED.
------------------------ ERROR OVER ------------------------
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/shmem.c

@@ -709,12 +709,12 @@ static int dump_one_shmem(struct shmem_info *si)
 
 		pgaddr = (unsigned long)addr + pfn * PAGE_SIZE;
 again:
-		if (xfer.parent && page_in_parent(pgstate == PST_DIRTY))
+		if (pgstate == PST_ZERO)
+			ret = 0;
+		else if (xfer.parent && page_in_parent(pgstate == PST_DIRTY))
 			ret = page_pipe_add_hole(pp, pgaddr);
-		else if (pgstate != PST_ZERO)
-			ret = page_pipe_add_page(pp, pgaddr);
 		else
-			ret = 0;
+			ret = page_pipe_add_page(pp, pgaddr);
 
 		if (ret == -EAGAIN) {
 			ret = dump_pages(pp, &xfer, addr);