[SCTP] Don't disable PMTU discovery when mtu is small

Right now, when we receive a mtu estimate smaller then minim
threshold in the ICMP message, we disable the path mtu discovery
on the transport.  This leads to the never increasing sctp fragmentation
point even when the real path mtu has increased.
Signed-off-by: NVlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/transport.c

@@ -271,8 +271,6 @@ void sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
 		 * pmtu discovery on this transport.
 		 */
 		t->pathmtu = SCTP_DEFAULT_MINSEGMENT;
-		t->param_flags = (t->param_flags & ~SPP_PMTUD) |
-			SPP_PMTUD_DISABLE;
 	} else {
 		t->pathmtu = pmtu;
 	}