s390/pgtable: add REGION3_KERNEL_EXEC protection

Similar to existing PAGE_KERNEL_EXEC and SEGMENT_KERNEL_EXEC
memory protection add REGION3_KERNEL_EXEC attribute that
could be set on PUD pgtable entries.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>