KVM: s390/mm: fix up indentation of set_guest_storage_key

commit ab3f285f ("KVM: s390/mm: try a cow on read only pages for
key ops")' misaligned a code block. Let's fixup the indentation.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>