[IPV6]: Check output fragmentation using dst_pmtu not dev->mtu.

net/ipv6/ip6_output.c

@@ -141,7 +141,7 @@ int ip6_output2(struct sk_buff *skb)
 
 int ip6_output(struct sk_buff *skb)
 {
-	if ((skb->len > skb->dst->dev->mtu || skb_shinfo(skb)->frag_list))
+	if ((skb->len > dst_pmtu(skb->dst) || skb_shinfo(skb)->frag_list))
 		return ip6_fragment(skb, ip6_output2);
 	else
 		return ip6_output2(skb);