s390/mm: modify pmdp_invalidate to return old value.

It's required to avoid losing dirty and accessed bits.

Link: http://lkml.kernel.org/r/20171213105756.69879-8-kirill.shutemov@linux.intel.com


Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>