[PATCH BUG-FIX] ipv6: allow to send packet after receiving ICMPv6

[Shan Wei <shanwei@cn.fujitsu.com>]

According to RFC2460, PMTU is set to the IPv6 Minimum Link
MTU (1280) and a fragment header should always be included
after a node receiving Too Big message reporting PMTU is
less than the IPv6 Minimum Link MTU.

After receiving a ICMPv6 Too Big message reporting PMTU is
less than the IPv6 Minimum Link MTU, sctp *can't* send any
data/control chunk that total length including IPv6 head 
and IPv6 extend head is less than IPV6_MIN_MTU(1280 bytes).

The failure occured in p6_fragment(), about reason 
see following(take SHUTDOWN chunk for example):
sctp_packet_transmit (SHUTDOWN chunk, len\x16 byte)
|------sctp_v6_xmit (local_df=0)
   |------ip6_xmit
       |------ip6_output (dst_allfrag is ture)
           |------ip6_fragment

In ip6_fragment(), for local_df=0, drops the the packet
and returns EMSGSIZE.

The patch fixes it with adding check length of skb->len.
In this case, Ipv6 not to fragment upper protocol data,
just only add a fragment header before it. 

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
---
 net/ipv6/ip6_output.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 5129a16..c3edb6c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -625,7 +625,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	/* We must not fragment if the socket is set to force MTU discovery
 	 * or if the skb it not generated by a local socket.
 	 */
-	if (!skb->local_df) {
+	if (!skb->local_df && skb->len > mtu) {
 		skb->dev = skb_dst(skb)->dev;
 		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
 		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
--
1.6.3.3