KVM: s390: use WARN_ON_ONCE only for checking

Move the real logic that always has to be executed out of the
WARN_ON_ONCE.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170830160603.5452-3-david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>