s390/mm: implement set_memory_4k()

Implement set_memory_4k() which will split any present large or huge
mapping in the given range to a 4k mapping.

Link: https://lore.kernel.org/r/20210728190254.3921642-2-hca@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>