net: tcp: Fix ACK check in LAST_ACK state

The final ACK check during passive close was wrong - we should not
compare its SEQ number with the ACK number we've sent last, but rather
compare the ACK number it acknowledges matches our current SEQ number on
the connection. This ensures, that the ACK received is really
acknowledging the FIN packet we've sent from our side, and is not just
some earlier retransmission. Currently the latter could be the case, and
we've closed the connection prematurely. In result, when the real "final
ACK" arrived, the TCP stack replied with RST.

Subsequently, we should increment the SEQ number on the connection after
sending FIN packet, so that we are able to identify final ACK correctly,
just as it's done in active close cases.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>