[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/output.c b/net/dccp/output.c
index 0994b13..ef22f3c 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -125,16 +125,7 @@
 
 		memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
 		err = icsk->icsk_af_ops->queue_xmit(skb, sk, 0);
-		if (err <= 0)
-			return err;
-
-		/* NET_XMIT_CN is special. It does not guarantee,
-		 * that this packet is lost. It tells that device
-		 * is about to start to drop packets or already
-		 * drops some packets of the same priority and
-		 * invokes us to send less aggressively.
-		 */
-		return err == NET_XMIT_CN ? 0 : err;
+		return net_xmit_eval(err);
 	}
 	return -ENOBUFS;
 }
@@ -426,8 +417,7 @@
 		if (skb != NULL) {
 			memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
 			err = inet_csk(sk)->icsk_af_ops->queue_xmit(skb, sk, 0);
-			if (err == NET_XMIT_CN)
-				err = 0;
+			return net_xmit_eval(err);
 		}
 	}