sctp: flush if we can't fit another DATA chunk

There is no point on delaying the packet if we can't fit a single byte
of data on it anymore. So lets just reduce the threshold by the amount
that a data chunk with 4 bytes (rounding) would use.

v2: based on the right tree

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/output.c

@@ -705,7 +705,8 @@ static sctp_xmit_t sctp_packet_can_append_data(struct sctp_packet *packet,
 	/* Check whether this chunk and all the rest of pending data will fit
 	 * or delay in hopes of bundling a full sized packet.
 	 */
-	if (chunk->skb->len + q->out_qlen >= transport->pathmtu - packet->overhead)
+	if (chunk->skb->len + q->out_qlen >
+		transport->pathmtu - packet->overhead - sizeof(sctp_data_chunk_t) - 4)
 		/* Enough data queued to fill a packet */
 		return SCTP_XMIT_OK;