[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/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 9a8e690..9a88395 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -502,8 +502,7 @@
 
 		ipv6_addr_copy(&fl.fl6_dst, &treq->rmt_addr);
 		err = ip6_xmit(sk, skb, &fl, opt, 0);
-		if (err == NET_XMIT_CN)
-			err = 0;
+		err = net_xmit_eval(err);
 	}
 
 done: