s390/kasan: cleanup setup of zero pgtable

Fix variables initialization coding style and setup zero
pgtable same way region and segment pgtables are set up.

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>