kdat: Handle pagemaps with zeroed pfns

Recent kernels allow for user to read proc pagemap file, but zero
pfns in it. Support this mode for user dumps.

https://github.com/xemul/criu/issues/101



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