s390/mm: add set_pXd()/set_pte() helper functions

Add set_pXd()/set_pte() helper functions which must be used to update
page table entries. The new helpers use WRITE_ONCE() to make sure that
a page table entry is written to only once.

Without this the compiler could otherwise generate code which writes
several times to a page table entry when updating its contents from
invalid to valid, which could lead to surprising results especially
for multithreaded processes...

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
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>