sctp: sctp_transport_dst_check should check if transport pmtu is dst mtu

Now when sending a packet, sctp_transport_dst_check will check if dst
is obsolete by calling ipv4/ip6_dst_check. But they return obsolete
only when adding a new cache, after that when the cache's pmtu is
updated again, it will not trigger transport->dst/pmtu's update.

It can be reproduced by reducing route's pmtu twice. At the 1st time
client will add a new cache, and transport->pathmtu gets updated as
sctp_transport_dst_check finds it's obsolete. But at the 2nd time,
cache's mtu is updated, sctp client will never send out any packet,
because transport->pmtu has no chance to update.

This patch is to fix this by also checking if transport pmtu is dst
mtu in sctp_transport_dst_check, so that transport->pmtu can be
updated on time.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -596,7 +596,9 @@ 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, t->dst_cookie))
+	if (t->dst && (!dst_check(t->dst, t->dst_cookie) ||
+		       t->pathmtu != max_t(size_t, SCTP_TRUNC4(dst_mtu(t->dst)),
+					   SCTP_DEFAULT_MINSEGMENT)))
 		sctp_transport_dst_release(t);
 
 	return t->dst;