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 obviously not to include linux/mm.h in our pgtable.h, which
unbreaks the build.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>