[CCID2]: Sequence number wraparound issues

This replaces several uses of standard arithmetic with the DCCP
sequence number arithmetic functions. The problem here is that the
sequence number wrap-around was not taken into consideration.

 * Condition "seqp->ccid2s_seq <= prev->ccid2s_seq" has been replaced
   by

   	dccp_delta_seqno(seqp->ccid2s_seq, prev->ccid2s_seq) >= 0

   since if seqp is `before' prev, then the delta_seqno() is positive.

 * The test whether sequence numbers `a' and `b' are consecutive has
   the form

   	dccp_delta_seqno(a, b) == 1

 * Increment of ccid2hctx_rpseq could be done using dccp_inc_seqno(),
   but since here the incremented ccid2hctx_rpseq == seqno, used
   assignment instead.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dccp/ccids/ccid2.c

@@ -59,7 +59,8 @@ static void ccid2_hc_tx_check_sanity(const struct ccid2_hc_tx_sock *hctx)
 				pipe++;
 
 			/* packets are sent sequentially */
-			BUG_ON(seqp->ccid2s_seq <= prev->ccid2s_seq);
+			BUG_ON(dccp_delta_seqno(seqp->ccid2s_seq,
+						prev->ccid2s_seq ) >= 0);
 			BUG_ON(time_before(seqp->ccid2s_sent,
 					   prev->ccid2s_sent));
 
@@ -562,8 +563,8 @@ static void ccid2_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
 		hctx->ccid2hctx_rpseq = seqno;
 	} else {
 		/* check if packet is consecutive */
-		if ((hctx->ccid2hctx_rpseq + 1) == seqno)
-			hctx->ccid2hctx_rpseq++;
+		if (dccp_delta_seqno(hctx->ccid2hctx_rpseq, seqno) == 1)
+			hctx->ccid2hctx_rpseq = seqno;
 		/* it's a later packet */
 		else if (after48(seqno, hctx->ccid2hctx_rpseq)) {
 			hctx->ccid2hctx_rpdupack++;