s390/uv: gmap_make_secure() cleanups for further changes

Let's factor out handling of LRU cache draining and convert the if-else
chain to a switch-case.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-3-david@redhat.com


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