KVM: use rcu access function for irq routing

irq routing is rcu protected. Use the proper access functions.
Found by sparse

virt/kvm/irqchip.c:233:13: warning: incorrect type in assignment (different address spaces)
virt/kvm/irqchip.c:233:13:    expected struct kvm_irq_routing_table *old
virt/kvm/irqchip.c:233:13:    got struct kvm_irq_routing_table [noderef] <asn:4>*irq_routing

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/irqchip.c

@@ -230,7 +230,7 @@ int kvm_set_irq_routing(struct kvm *kvm,
 	}
 
 	mutex_lock(&kvm->irq_lock);
-	old = kvm->irq_routing;
+	old = rcu_dereference_protected(kvm->irq_routing, 1);
 	rcu_assign_pointer(kvm->irq_routing, new);
 	kvm_irq_routing_update(kvm);
 	kvm_arch_irq_routing_update(kvm);