When forwarding TCP packets, the internal tcpiphdr struct length was wrongly
used inside the IP header. This commit changes the behaviour to what is used
by tcp_output.c, using the correct full IP header + payload length.
Signed-off-by: NSjors Gielen <sjors@sjorsgielen.nl>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>