[TCP/DCCP]: Introduce net_xmit_eval

Throughout the TCP/DCCP (and tunnelling) code, it often happens that the
return code of a transmit function needs to be tested against NET_XMIT_CN
which is a value that does not indicate a strict error condition.

This patch uses a macro for these recurring situations which is consistent
with the already existing macro net_xmit_errno, saving on duplicated code.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
index bc400b2..61c0901 100644
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -501,8 +501,7 @@
 		err = ip_build_and_send_pkt(skb, sk, ireq->loc_addr,
 					    ireq->rmt_addr,
 					    ireq->opt);
-		if (err == NET_XMIT_CN)
-			err = 0;
+		err = net_xmit_eval(err);
 	}
 
 out:
@@ -571,7 +570,7 @@
 				    rxskb->nh.iph->saddr, NULL);
 	bh_unlock_sock(dccp_v4_ctl_socket->sk);
 
-	if (err == NET_XMIT_CN || err == 0) {
+	if (net_xmit_eval(err) == 0) {
 		DCCP_INC_STATS_BH(DCCP_MIB_OUTSEGS);
 		DCCP_INC_STATS_BH(DCCP_MIB_OUTRSTS);
 	}