net: tcp: When closing the connection send FIN without extra delays

The earlier code was always queuing the FIN that is sent when
connection is closed. This caused long delay (200 ms) before the peer at
the other end noticed that the connection was actually closed.
Now check if there is nothing in the queue, then send the FIN
immediately. If there is some data in the queue, flush it when a valid
ack has been received.

Fixes #19678

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>