s390x/kvm: use kvm_gsi_routing_enabled in flic

Let's use kvm_gsi_routing_enabled() to check if kvm supports
KVM_CAP_IRQ_ROUTING in order to avoid a needless ioctl invocation.

Signed-off-by: Fei Li <sherrylf@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
