s390/mm: use set_memory_*() helpers instead of open coding

Given that set_memory_rox() and set_memory_rwnx() exist, it is possible
to get rid of all open coded __set_memory() usages and replace them with
proper helper calls everywhere.

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>