[IPV6]: MTU discovery check in ip6_fragment()

Adds a check in ip6_fragment() mirroring ip_fragment() for packets
that we can't fragment, and sends an ICMP Packet Too Big message
in response.
Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -567,6 +567,19 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	nexthdr = *prevhdr;
 
 	mtu = dst_mtu(&rt->u.dst);
+
+	/* We must not fragment if the socket is set to force MTU discovery
+	 * or if the skb it not generated by a local socket.  (This last
+	 * check should be redundant, but it's free.)
+	 */
+	if (!np || np->pmtudisc >= IPV6_PMTUDISC_DO) {
+		skb->dev = skb->dst->dev;
+		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu, skb->dev);
+		IP6_INC_STATS(ip6_dst_idev(skb->dst), IPSTATS_MIB_FRAGFAILS);
+		kfree_skb(skb);
+		return -EMSGSIZE;
+	}
+
 	if (np && np->frag_size < mtu) {
 		if (np->frag_size)
 			mtu = np->frag_size;