netfilter: synproxy: fix rst sequence number mismatch

14:51:00.024418 IP 192.168.122.1.41462 > netfilter.90: Flags [S], seq
4023580551,
14:51:00.024454 IP netfilter.90 > 192.168.122.1.41462: Flags [S.], seq
727560212, ack 4023580552,
14:51:00.024524 IP 192.168.122.1.41462 > netfilter.90: Flags [.], ack 1,

Note: here, synproxy will send a SYN to the real server, as the 3whs was
completed sucessfully. Instead of a syn/ack that we can intercept, we instead
received a reset packet from the real backend, that we forward to the original
client. However, we don't use the correct sequence number, so the reset is not
effective in closing the connection coming from the client.

14:51:00.024550 IP netfilter.90 > 192.168.122.1.41462: Flags [R.], seq
3567407084,
14:51:00.231196 IP 192.168.122.1.41462 > netfilter.90: Flags [.], ack 1,
14:51:00.647911 IP 192.168.122.1.41462 > netfilter.90: Flags [.], ack 1,
14:51:01.474395 IP 192.168.122.1.41462 > netfilter.90: Flags [.], ack 1,

Fixes: 48b1de4c ("netfilter: add SYNPROXY core/target")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>