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

arch/s390/mm/gmap.c

@@ -788,19 +788,19 @@ static inline unsigned long *gmap_table_walk(struct gmap *gmap,
 					     unsigned long gaddr, int level)
 {
 	const int asce_type = gmap->asce & _ASCE_TYPE_MASK;
-	unsigned long *table;
+	unsigned long *table = gmap->table;
 
-	if ((gmap->asce & _ASCE_TYPE_MASK) + 4 < (level * 4))
-		return NULL;
 	if (gmap_is_shadow(gmap) && gmap->removed)
 		return NULL;
 
+	if (WARN_ON_ONCE(level > (asce_type >> 2) + 1))
+		return NULL;
+
 	if (asce_type != _ASCE_TYPE_REGION1 &&
 	    gaddr & (-1UL << (31 + (asce_type >> 2) * 11)))
 		return NULL;
 
-	table = gmap->table;
-	switch (gmap->asce & _ASCE_TYPE_MASK) {
+	switch (asce_type) {
 	case _ASCE_TYPE_REGION1:
 		table += (gaddr & _REGION1_INDEX) >> _REGION1_SHIFT;
 		if (level == 4)