net: tcp: Handle special case where accepted socket is closed

Handle this corner case with TCP connection closing:

1) Client A connects, it is accepted and can send data to us
2) Client B connects, the application needs to call accept()
   before we will receive any data from client A to the application.
   The app has not yet called accept() at this point (for
   whatever reason).
3) Client B then disconnects and we receive FIN. The connection
   cleanup is a bit tricky as the client is in half-connected state
   meaning that the connection is in established state but the
   accept_q in socket queue contains still data which needs to be
   cleared.
4) Client A then disconnects, all data is sent etc

The above was not working correctly as the system did not handle the
step 3) properly. The client B was accepted in the application even
if the connection was closing.

After this commit, the commit called "net: tcp: Accept connections
only in LISTENING state" and related other commits are no longer
needed and are reverted.

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