net: tcp: When sending FIN, make sure it goes with ACK and proper seq

Without change to add ACK to FIN, invalid TCP packet is generated,
where ack sequence number is non-zero. Without adjusting sequence
number as done, ACK which we send in response to peer's FIN/ACK is
not recognized by peer, and peer keeps retransmitting its FIN/ACK.

Jira: ZEP-2104

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