s390/kfence: Split kfence pool into 4k mappings in arch_kfence_init_pool()

Since commit d08d4e7c ("s390/mm: use full 4KB page for 2KB PTE"),
there is no longer any reason to avoid splitting the kfence pool into
4k mappings in arch_kfence_init_pool(). Remove the architecture-specific
kfence_split_mapping().

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>