s390/mm: implement set_memory_rox()

Provide the s390 specific native set_memory_rox() implementation to avoid
frequent set_memory_ro(); set_memory_x() call pairs.

This is the s390 variant of commit 60463628 ("x86/mm: Implement native
set_memory_rox()").

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>