Revert "sctp: remove sctp_transport_pmtu_check"

This reverts commit 22d7be26.

The dst's mtu in transport can be updated by a non sctp place like
in xfrm where the MTU information didn't get synced between asoc,
transport and dst, so it is still needed to do the pmtu check
in sctp_packet_config.
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -608,4 +608,16 @@ static inline __u32 sctp_dst_mtu(const struct dst_entry *dst)
 				 SCTP_DEFAULT_MINSEGMENT));
 }
 
+static inline bool sctp_transport_pmtu_check(struct sctp_transport *t)
+{
+	__u32 pmtu = sctp_dst_mtu(t->dst);
+
+	if (t->pathmtu == pmtu)
+		return true;
+
+	t->pathmtu = pmtu;
+
+	return false;
+}
+
 #endif /* __net_sctp_h__ */

net/sctp/output.c

@@ -118,6 +118,9 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 		sctp_transport_route(tp, NULL, sp);
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
 			sctp_assoc_sync_pmtu(asoc);
+	} else if (!sctp_transport_pmtu_check(tp)) {
+		if (asoc->param_flags & SPP_PMTUD_ENABLE)
+			sctp_assoc_sync_pmtu(asoc);
 	}
 
 	if (asoc->pmtu_pending) {