s390/mm: convert pte_val()/pXd_val() into functions

Disallow constructs like this:

        pte_val(*pte) = __pa(addr) | prot;

which would directly write into a page table. Users are supposed to
use the set_pte()/set_pXd() primitives, which guarantee block
concurrent (aka atomic) writes.

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>