s390/mcck: fix invalid KVM guest condition check

Wrong condition check is used to decide if a machine check hit
while in KVM guest. As result of this check the instruction
following the SIE critical section might be considered as still
in KVM guest and _CIF_MCCK_GUEST CPU flag mistakenly set as
result.

Fixes: c929500d

 ("s390/nmi: s390: New low level handling for machine check happening in guest")
Cc: <stable@vger.kernel.org>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>