DESC
remap_file_pages protection support: avoid redundant pte_file PTE's
EDESC
From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

For linear VMA's, there is no need to install pte_file PTEs to remember the
offset. We could probably go as far as checking directly the address and
protection like in include/linux/pagemap.h:set_nonlinear_pte(), instead of
vma->vm_flags. Also add some warnings on the path which used to cope with such
PTE's.

Untested yet.
