s390/mcck: optimize user mode check in case of !CONFIG_KVM

In case of the !CONFIG_KVM use "jz" instead of "jnz" when
detecting user mode and get rid of unnecessary jump as result.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Christia Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>