KVM: s390: Use insn_length() to calculate length of instruction

The common s390 function insn_length() results in slightly smaller
(and thus hopefully faster) code than the calculation of the
instruction length via a lookup-table. So let's use that function
in the interrupt delivery code, too.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>