[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/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f63e99a..6a8581a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -484,13 +484,7 @@
 
 	tcp_enter_cwr(sk);
 
-	/* 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);
 
 #undef SYSCTL_FLAG_TSTAMPS
 #undef SYSCTL_FLAG_WSCALE