[IPV6]: In ndisc_netdev_event, handle NETDEV_DOWN.

Based upon patches and commentary from Thomas Spatzier,
Stephen Hemminger, and Andi Kleen.

net/ipv6/ndisc.c

@@ -1403,6 +1403,10 @@ static int ndisc_netdev_event(struct notifier_block *this, unsigned long event,
 		neigh_changeaddr(&nd_tbl, dev);
 		fib6_run_gc(0);
 		break;
+	case NETDEV_DOWN:
+		neigh_ifdown(&nd_tbl, dev);
+		fib6_run_gc(0);
+		break;
 	default:
 		break;
 	}