s390/mm: Convert to LOCK_MM_AND_FIND_VMA

With the gmap code gone s390 can be easily converted to
LOCK_MM_AND_FIND_VMA like it has been done for most other
architectures.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/20241022120601.167009-12-imbrenda@linux.ibm.com


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