SCTP: Initialize partial_bytes_acked to 0, when all of the data is ac…

…ked.

According to RFC4960 7.2.2, 
When all of the data transmitted by the sender has
been acknowledged by the recerver, partial_bytes_acked is initialized to 0.

This patch conforms to rfc requirement. 
Without this fix, cwnd might be error incremented.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/outqueue.c

@@ -1532,6 +1532,8 @@ static void sctp_check_transmitted(struct sctp_outq *q,
 						  bytes_acked);
 
 			transport->flight_size -= bytes_acked;
+			if (transport->flight_size == 0)
+				transport->partial_bytes_acked = 0;
 			q->outstanding_bytes -= bytes_acked;
 		} else {
 			/* RFC 2960 6.1, sctpimpguide-06 2.15.2