mm: pagewalk: call pte_hole() for VM_PFNMAP during walk_page_range

walk_page_range silently skips vma having VM_PFNMAP set,
which leads to undesirable behaviour at client end (who
called walk_page_range). For example for pagemap_read,
when no callbacks are called against VM_PFNMAP vma,
pagemap_read may prepare pagemap data at wrong index.

Change-Id: I057b5c8ede1ae4bb9e3f8639e10bd4fcbf23da7e
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>