[IPV6]: Fix offset of payload with extension header.

Based upon a patch from Thomas Graf <tgraf@suug.ch>.

net/ipv6/ip6_output.c

@@ -1247,11 +1247,9 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to, int offse
 		inet->cork.length = 0;
 		inet->sndmsg_page = NULL;
 		inet->sndmsg_off = 0;
-		if ((exthdrlen = rt->u.dst.header_len) != 0) {
-			length += exthdrlen;
-			transhdrlen += exthdrlen;
-		}
-		exthdrlen += opt ? opt->opt_flen : 0;
+		exthdrlen = rt->u.dst.header_len + (opt ? opt->opt_flen : 0);
+		length += exthdrlen;
+		transhdrlen += exthdrlen;
 	} else {
 		rt = np->cork.rt;
 		if (inet->cork.flags & IPCORK_OPT)