KVM: make kvm_unregister_irq_ack_notifier() safe

We never pass a NULL notifier pointer here, but we may well
pass a notifier struct which hasn't previously been
registered.

Guard against this by using hlist_del_init() which will
not do anything if the node hasn't been added to the list
and, when removing the node, will ensure that a subsequent
call to hlist_del_init() will be fine too.

Fixes an oops seen when an assigned device is freed before
and IRQ is assigned to it.
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/irq_comm.c

@@ -63,9 +63,7 @@ void kvm_register_irq_ack_notifier(struct kvm *kvm,
 
 void kvm_unregister_irq_ack_notifier(struct kvm_irq_ack_notifier *kian)
 {
-	if (!kian)
-		return;
-	hlist_del(&kian->link);
+	hlist_del_init(&kian->link);
 }
 
 /* The caller must hold kvm->lock mutex */