net: tcp: Properly queue FIN packets for retransmission

In TCP protocol, any packet is subject to retransmission if not
ACKed in expected time. Thus, any packet, including FIN (and SYN
for that matter) should be added to the retransmission queue.

In our case, despite its name, queue_fin() function didn't add
FIN packet to rexmit queue, so do that. Then, in
net_tcp_ack_received() which handles ACKs, make sure that we can
handle FIN packets: calculate its sequence number properly, don't
make adhoc adjustments to retransmission logic (it's handled
centrally in restart_timer() already), etc.

Fixes: #8188

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>