KVM: s390: fast path for shadow gmaps in gmap notifier

The default kvm gmap notifier doesn't have to handle shadow gmaps.
So let's just directly exit in case we get notified about one.

Acked-by: Christian Borntraeger <[e-mail]>
Signed-off-by: David Hildenbrand <[e-mail]>
Signed-off-by: Christian Borntraeger <[e-mail]>

arch/s390/kvm/kvm-s390.c

···
 	unsigned long prefix;
 	int i;
 
+	if (gmap_is_shadow(gmap))
+		return;
 	if (start >= 1UL << 31)
 		/* We are only interested in prefix pages */
 		return;