mm, thp: Do not make page table dirty unconditionally in follow_trans_huge_pmd()

mm, thp: Do not make page table dirty unconditionally in follow_trans_huge_pmd()

Currently, we unconditionally make page table dirty in
follow_trans_huge_pmd(). However, it should be set only in case of
write access.

This is port of upstream commit: a8f97366452ed491d13cf1e44241bc0b5740b1f0

Orabug: 27165913

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2017-1000405
Signed-off-by: Nitin Gupta <nitin.m.gupta@oracle.com>
Reviewed-by: Dhaval Giani <dhaval.giani@oracle.com>