sctp: process fwd tsn chunk only when prsctp is enabled

This patch is to check if asoc->peer.prsctp_capable is set before
processing fwd tsn chunk, if not, it will return an ERROR to the
peer, just as rfc3758 section 3.3.1 demands.

Reported-by: Julian Cordes <julian.cordes@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/sm_statefuns.c

@@ -3867,6 +3867,9 @@ sctp_disposition_t sctp_sf_eat_fwd_tsn(struct net *net,
 		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
 	}
 
+	if (!asoc->peer.prsctp_capable)
+		return sctp_sf_unk_chunk(net, ep, asoc, type, arg, commands);
+
 	/* Make sure that the FORWARD_TSN chunk has valid length.  */
 	if (!sctp_chunk_length_valid(chunk, sizeof(struct sctp_fwdtsn_chunk)))
 		return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
@@ -3935,6 +3938,9 @@ sctp_disposition_t sctp_sf_eat_fwd_tsn_fast(
 		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
 	}
 
+	if (!asoc->peer.prsctp_capable)
+		return sctp_sf_unk_chunk(net, ep, asoc, type, arg, commands);
+
 	/* Make sure that the FORWARD_TSN chunk has a valid length.  */
 	if (!sctp_chunk_length_valid(chunk, sizeof(struct sctp_fwdtsn_chunk)))
 		return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,