Revert "mm/pagewalk.c: walk_page_range should avoid VM_PFNMAP areas"

Comments from Marc vdW:
- This seems to fix those crazy memory corruption issues, often shown wh
  inodes are NULL and the kernel panics. Test by triggering a bug report in
  development options before/after.
- This is an important change as minor bugs that usually just trigger a bug
  report lead to reboots for no apparent reason, masking the real issue.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>