ipv6: flush nd cache on IFF_NOARP change

This patch is the IPv6 equivalent of commit
6c8b4e3ff81b ("arp: flush arp cache on IFF_NOARP change")

Without it, we keep buggy neighbours in the cache, with destination
MAC address equal to our own MAC address.

Tested:
 tcpdump -i eth0 -s 0 ip6 -n -e &
 ip link set dev eth0 arp off
 ping6 remote   // sends buggy frames
 ip link set dev eth0 arp on
 ping6 remote   // should work once kernel is patched

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Mario Fanelli <mariofanelli@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1650,6 +1650,7 @@ int ndisc_rcv(struct sk_buff *skb)
 static int ndisc_netdev_event(struct notifier_block *this, unsigned long event, void *ptr)
 {
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
+	struct netdev_notifier_change_info *change_info;
 	struct net *net = dev_net(dev);
 	struct inet6_dev *idev;
 
@@ -1664,6 +1665,11 @@ static int ndisc_netdev_event(struct notifier_block *this, unsigned long event,
 			ndisc_send_unsol_na(dev);
 		in6_dev_put(idev);
 		break;
+	case NETDEV_CHANGE:
+		change_info = ptr;
+		if (change_info->flags_changed & IFF_NOARP)
+			neigh_changeaddr(&nd_tbl, dev);
+		break;
 	case NETDEV_DOWN:
 		neigh_ifdown(&nd_tbl, dev);
 		fib6_run_gc(0, net, false);