CRIU doesn't save vaddr of each anon shmem page in anon shared mem pagemap img.
It saves page offset from the beginning of anon shared memory area.
CRIU calls page_xfer_dump_pages() with non zero @off argument
to convert dumper virtual addresses to such offsets.

The problem is in page_xfer_dump_pages() code. It substracts @off
only for pages in pagemap but not for holes in pagemap.
Bug is fixed in this patch.
This patch is just a copy-paste of valid code path for pages to code path for holes.

Bug is not currently reproduced in CRIU because:
1. Only anon shmem provides non-zero @off value to page_xfer_dump_pages()
2. Anon shared memory doesn't create holes in its pagemap (for now)

This bugfix is a preparation for anon shared memory deduplication patchset.
Signed-off-by: Fyodor Bocharov <fbocharov@yandex.ru>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>