s390/uv: Split large folios in gmap_make_secure()

While s390x makes sure to never have PMD-mapped THP in processes that use
KVM -- by remapping them using PTEs in
thp_split_walk_pmd_entry()->split_huge_pmd() -- there is still the
possibility of having PTE-mapped THPs (large folios) mapped into guest
memory.

This would happen if user space allocates memory before calling
KVM_CREATE_VM (which would call s390_enable_sie()). With upstream QEMU,
this currently doesn't happen, because guest memory is setup and
conditionally preallocated after KVM_CREATE_VM.

Could it happen with shmem/file-backed memory when another process
allocated memory in the pagecache? Likely, although currently not a
common setup.

Trying to split any PTE-mapped large folios sounds like the right and
future-proof thing to do here. So let's call split_folio() and handle the
return values accordingly.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-4-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

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