sctp: Drop ICMP packet too big message with MTU larger than current PMTU

If ICMP packet too big message is received with MTU larger than current
PMTU, SCTP will still accept this ICMP message and sync the PMTU of assoc
with the wrong MTU.

Endpoing A                 Endpoint B
(ESTABLISHED)              (ESTABLISHED)
ICMP         --------->
(packet too big, MTU too larger)
                           sync PMTU

This patch fixed the problem by drop that ICMP message.
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: NVlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -369,7 +369,7 @@ static void sctp_add_backlog(struct sock *sk, struct sk_buff *skb)
 void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 			   struct sctp_transport *t, __u32 pmtu)
 {
-	if (!t || (t->pathmtu == pmtu))
+	if (!t || (t->pathmtu <= pmtu))
 		return;
 
 	if (sock_owned_by_user(sk)) {