s390/mm: use set_pXd()/set_pte() helper functions everywhere

Use the new set_pXd()/set_pte() helper functions at all places where
page table entries are modified.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>