[IPV6]: Add ndisc_netdev_notifier unregister.

If inet6_init() fails later than ndisc_init() call, or IPv6 module is
unloaded, ndisc_netdev_notifier call remains in the list and will follows in
oops later.
Signed-off-by: Dmitry Mishin <dim@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1742,6 +1742,7 @@ int __init ndisc_init(struct net_proto_family *ops)
 
 void ndisc_cleanup(void)
 {
+	unregister_netdevice_notifier(&ndisc_netdev_notifier);
 #ifdef CONFIG_SYSCTL
 	neigh_sysctl_unregister(&nd_tbl.parms);
 #endif