sctp: Don't lookup dst if transport dst is still valid

When sctp sits on IPv6, sctp_transport_dst_check pass cookie as ZERO,
as a result ip6_dst_check always fail out. This behaviour makes
transport->dst useless, because every sctp_packet_transmit must look
for valid dst.

Add a dst_cookie into sctp_transport, and set the cookie whenever we
get new dst for sctp_transport. So dst validness could be checked
against it.

Since I have split genid for IPv4 and IPv6, also delete/add IPv6 address
will also bump IPv6 genid. So issues we discussed in:
http://marc.info/?l=linux-netdev&m=137404469219410&w=4
have all been sloved for this patch.
Signed-off-by: Fan Du <fan.du@windriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -613,7 +613,7 @@ static inline void sctp_v4_map_v6(union sctp_addr *addr)
  */
 static inline struct dst_entry *sctp_transport_dst_check(struct sctp_transport *t)
 {
-	if (t->dst && !dst_check(t->dst, 0)) {
+	if (t->dst && !dst_check(t->dst, t->dst_cookie)) {
 		dst_release(t->dst);
 		t->dst = NULL;
 	}

include/net/sctp/structs.h

@@ -946,6 +946,7 @@ struct sctp_transport {
 	__u64 hb_nonce;
 
 	struct rcu_head rcu;
+	u32 dst_cookie;
 };
 
 struct sctp_transport *sctp_transport_new(struct net *, const union sctp_addr *,

net/sctp/ipv6.c

@@ -351,7 +351,7 @@ static void sctp_v6_get_dst(struct sctp_transport *t, union sctp_addr *saddr,
 
 		rt = (struct rt6_info *)dst;
 		t->dst = dst;
-
+		t->dst_cookie = rt->rt6i_node ? rt->rt6i_node->fn_sernum : 0;
 		pr_debug("rt6_dst:%pI6 rt6_src:%pI6\n", &rt->rt6i_dst.addr,
 			 &fl6->saddr);
 	} else {