[IPV6]: Do not ignore IPV6_MTU socket option.

Based on patch by Hoerdt Mickael <hoerdt@clarinet.u-strasbg.fr>.
Signed-off-by: YOSHIFUJI Hideaki <yosufuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -494,6 +494,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	struct net_device *dev;
 	struct sk_buff *frag;
 	struct rt6_info *rt = (struct rt6_info*)skb->dst;
+	struct ipv6_pinfo *np = skb->sk ? inet6_sk(skb->sk) : NULL;
 	struct ipv6hdr *tmp_hdr;
 	struct frag_hdr *fh;
 	unsigned int mtu, hlen, left, len;
@@ -505,7 +506,12 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	hlen = ip6_find_1stfragopt(skb, &prevhdr);
 	nexthdr = *prevhdr;
 
-	mtu = dst_mtu(&rt->u.dst) - hlen - sizeof(struct frag_hdr);
+	mtu = dst_mtu(&rt->u.dst);
+	if (np && np->frag_size < mtu) {
+		if (np->frag_size)
+			mtu = np->frag_size;
+	}
+	mtu -= hlen + sizeof(struct frag_hdr);
 
 	if (skb_shinfo(skb)->frag_list) {
 		int first_len = skb_pagelen(skb);
@@ -882,7 +888,12 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,
 		inet->cork.fl = *fl;
 		np->cork.hop_limit = hlimit;
 		np->cork.tclass = tclass;
-		inet->cork.fragsize = mtu = dst_mtu(rt->u.dst.path);
+		mtu = dst_mtu(rt->u.dst.path);
+		if (np && np->frag_size < mtu) {
+			if (np->frag_size)
+				mtu = np->frag_size;
+		}
+		inet->cork.fragsize = mtu;
 		if (dst_allfrag(rt->u.dst.path))
 			inet->cork.flags |= IPCORK_ALLFRAG;
 		inet->cork.length = 0;