selftests/mm: pagemap_scan ioctl: add PFN ZERO test cases

Add test cases to test the correctness of PFN ZERO flag of pagemap_scan
ioctl.  Test with normal pages backed memory and huge pages backed memory.

Link: https://lkml.kernel.org/r/20250707073321.106431-1-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>