s390/mm: implement set_memory_rwnx()

Given that set_memory_rox() is implemented, provide also set_memory_rwnx().
This allows to get rid of all open coded __set_memory() usages in s390
architecture code.

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>