ip6_gre: add the process for redirect in ip6gre_err

This patch is to add redirect icmp packet process for ip6gre by
calling ip6_redirect() in ip6gre_err(), as in vti6_err.

Prior to this patch, there's even no route cache generated after
receiving redirect.
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -369,6 +369,7 @@ static void ip6gre_tunnel_uninit(struct net_device *dev)
 static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		       u8 type, u8 code, int offset, __be32 info)
 {
+	struct net *net = dev_net(skb->dev);
 	const struct gre_base_hdr *greh;
 	const struct ipv6hdr *ipv6h;
 	int grehlen = sizeof(*greh);
@@ -442,6 +443,10 @@ static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 			mtu = IPV6_MIN_MTU;
 		t->dev->mtu = mtu;
 		return;
+	case NDISC_REDIRECT:
+		ip6_redirect(skb, net, skb->dev->ifindex, 0,
+			     sock_net_uid(net, NULL));
+		return;
 	}
 
 	if (time_before(jiffies, t->err_time + IP6TUNNEL_ERR_TIMEO))