net: sctp: sctp_assoc_control_transport: fix MTU size in SCTP_PF state

The SCTP Quick failover draft [1] section 5.1, point 5 says that the cwnd
should be 1 MTU. So, instead of 1, set it to 1 MTU.

  [1] https://tools.ietf.org/html/draft-nishida-tsvwg-sctp-failover-05Reported-by: NKarl Heiss <kheiss@gmail.com>
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Acked-by: NVlad Yasevich <vyasevich@gmail.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/associola.c

@@ -846,12 +846,12 @@ void sctp_assoc_control_transport(struct sctp_association *asoc,
 		else
 			spc_state = SCTP_ADDR_AVAILABLE;
 		/* Don't inform ULP about transition from PF to
-		 * active state and set cwnd to 1, see SCTP
+		 * active state and set cwnd to 1 MTU, see SCTP
 		 * Quick failover draft section 5.1, point 5
 		 */
 		if (transport->state == SCTP_PF) {
 			ulp_notify = false;
-			transport->cwnd = 1;
+			transport->cwnd = asoc->pathmtu;
 		}
 		transport->state = SCTP_ACTIVE;
 		break;