parisc: fix compile breakage caused by inlining maybe_mkwrite

On PARISC, we have an include of linux/mm.h inside our asm/pgtable.h, so
this patch

  commit 14fd403f
  Author: Andrea Arcangeli <aarcange@redhat.com>
  Date:   Thu Jan 13 15:46:37 2011 -0800

      thp: export maybe_mkwrite

causes us an unsatisfiable use of pte_mkwrite in linux/mm.h.

The fix is to avoid including linux/mm.h in our pgtable.h, which
unbreaks the build.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>