[DCCP]: Remove ambiguity in the way before48 is used

This removes two ambiguities in employing the new definition of before48,
following the analysis on http://www.mail-archive.com/dccp@vger.kernel.org/msg01295.html

 (1) Updating GSR when P.seqno >= S.SWL
     With the old definition we did not update when P.seqno and S.SWL are 2^47 apart. To
     ensure the same behaviour as with the old definition, this is replaced with the
     equivalent condition dccp_delta_seqno(S.SWL, P.seqno) >= 0

 (2) Sending SYNC when P.seqno >= S.OSR
     Here it is debatable whether the new definition causes an ambiguity: the case is
     similar to (1); and to have consistency with the case (1), we use the equivalent
     condition dccp_delta_seqno(S.OSR, P.seqno) >= 0

Detailed Justification

net/dccp/input.c

@@ -86,7 +86,8 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb)
 	    dh->dccph_type == DCCP_PKT_SYNCACK) {
 		if (between48(DCCP_SKB_CB(skb)->dccpd_ack_seq,
 			      dp->dccps_awl, dp->dccps_awh) &&
-		    !before48(DCCP_SKB_CB(skb)->dccpd_seq, dp->dccps_swl))
+		    dccp_delta_seqno(dp->dccps_swl,
+				     DCCP_SKB_CB(skb)->dccpd_seq) >= 0)
 			dccp_update_gsr(sk, DCCP_SKB_CB(skb)->dccpd_seq);
 		else
 			return -1;
@@ -203,7 +204,8 @@ static int __dccp_rcv_established(struct sock *sk, struct sk_buff *skb,
 		if (dp->dccps_role != DCCP_ROLE_CLIENT)
 			goto send_sync;
 check_seq:
-		if (!before48(DCCP_SKB_CB(skb)->dccpd_seq, dp->dccps_osr)) {
+		if (dccp_delta_seqno(dp->dccps_osr,
+				     DCCP_SKB_CB(skb)->dccpd_seq) >= 0) {
 send_sync:
 			dccp_send_sync(sk, DCCP_SKB_CB(skb)->dccpd_seq,
 				       DCCP_PKT_SYNC);