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

This reverts commit 62d4a5215cdd8d10c52c8fb2aa31c9e1a0b67fb0.

Comments from Marc vdW
- This seems to fix those crazy memory corruption issues, often shown when
  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.