s390/mm: split lowcore pages with set_memory_4k()

Use set_memory_4k() to split lowcore pages within the kernel mapping
instead of using the quite subtle !addr check within modify_pmd_table()
and modify_pud_table() to prevent large pages for address zero.

With this lowcore might be mapped with 1MB / 2GB frames and only later
will be split. This way this mapping is handled like every other.

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>