KVM: s390: fix fallthrough annotation

A case statement in kvm_s390_shadow_tables uses fallthrough annotations
which are not recognized by gcc because they are hidden within a block.
Move these annotations out of the block to fix (W=1) warnings like below:

arch/s390/kvm/gaccess.c: In function 'kvm_s390_shadow_tables':
arch/s390/kvm/gaccess.c:1029:26: warning: this statement may fall through [-Wimplicit-fallthrough=]
  case ASCE_TYPE_REGION1: {
                          ^

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>