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>