Fix total IP header length in forwarded TCP packets

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>

slirp/tcp_subr.c

@@ -204,7 +204,7 @@ tcp_respond(struct tcpcb *tp, struct tcpiphdr *ti, struct mbuf *m,
 	    m->m_len  -= sizeof(struct tcpiphdr) - sizeof(struct tcphdr)
 	                                         - sizeof(struct ip);
 	    ip = mtod(m, struct ip *);
-	    ip->ip_len = tlen;
+	    ip->ip_len = m->m_len;
 	    ip->ip_dst = tcpiph_save.ti_dst;
 	    ip->ip_src = tcpiph_save.ti_src;
 	    ip->ip_p = tcpiph_save.ti_pr;
@@ -224,7 +224,7 @@ tcp_respond(struct tcpcb *tp, struct tcpiphdr *ti, struct mbuf *m,
 	    m->m_len  -= sizeof(struct tcpiphdr) - sizeof(struct tcphdr)
 	                                         - sizeof(struct ip6);
 	    ip6 = mtod(m, struct ip6 *);
-	    ip6->ip_pl = tlen;
+	    ip6->ip_pl = tcpiph_save.ti_len;
 	    ip6->ip_dst = tcpiph_save.ti_dst6;
 	    ip6->ip_src = tcpiph_save.ti_src6;
 	    ip6->ip_nh = tcpiph_save.ti_nh6;