s390/mm/gmap: Refactor gmap_fault() and add support for pfault

s390/mm/gmap: Refactor gmap_fault() and add support for pfault

When specifying FAULT_FLAG_RETRY_NOWAIT as flag for gmap_fault(), the
gmap fault will be processed only if it can be resolved quickly and
without sleeping. This will be needed for pfault.

Refactor gmap_fault() to improve readability.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20241022120601.167009-4-imbrenda@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>