ipv6: Fix ip6_xmit to send fragments if ipfragok is true

SCTP used ip6_xmit() to send fragments after received ICMP packet too
big message. But while send packet used ip6_xmit, the skb->local_df is
not initialized. So when skb if enter ip6_fragment(), the following
code will discard the skb.

ip6_fragment(...)
{
    if (!skb->local_df) {
        ...
        return -EMSGSIZE;
    }
    ...
}

SCTP do the following step:
1. send packet ip6_xmit(skb, ipfragok=0)
2. received ICMP packet too big message
3. if PMTUD_ENABLE: ip6_xmit(skb, ipfragok=1)

This patch fixed the problem by set local_df if ipfragok is true.
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -236,6 +236,10 @@ int ip6_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl,
 	skb_reset_network_header(skb);
 	hdr = ipv6_hdr(skb);
 
+	/* Allow local fragmentation. */
+	if (ipfragok)
+		skb->local_df = 1;
+
 	/*
 	 *	Fill in the IPv6 header
 	 */