KVM: s390: vsie: gmap_table_walk() simplifications

Let's use asce_type where applicable. Also, simplify our sanity check for
valid table levels and convert it into a WARN_ON_ONCE(). Check if we even
have a valid gmap shadow as the very first step.

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200403153050.20569-6-david@redhat.com


Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>