s390/nmi: handle guarded storage validity failures for KVM guests

machine check validity bits reflect the state of the machine check. If a
guest does not make use of guarded storage, the validity bit might be
off. We can not use the host CR bit to decide if the validity bit must
be on. So ignore "invalid" guarded storage controls for KVM guests in
the host and rely on the machine check being forwarded to the guest.  If
no other errors happen from a host perspective everything is fine and no
process must be killed and the host can continue to run.

Cc: stable@vger.kernel.org
Fixes: c929500d ("s390/nmi: s390: New low level handling for machine check happening in guest")
Reported-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Tested-by: Carsten Otte <cotte@de.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>