TCP: attempt to flush the write buffers before closing

When a socket is closed, it should make sure that any pending write
data is sent before the FIN is sent.  It already would wait for all
sent data to be acked, however it would discard any pending write
data that had not been sent at least once.

This change adds a check for pending write data in addition to unacked
data.  However, to be able to actually send any new data, the send
callback must be left.  The callback should be freed later when the socket
is actually destroyed.

net/socket/net_close.c

@@ -223,7 +223,7 @@ static uint16_t netclose_interrupt(FAR struct net_driver_s *dev,
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
   /* Check if all outstanding bytes have been ACKed */
 
-  else if (conn->unacked != 0)
+  else if (conn->unacked != 0 || !sq_empty(&conn->write_q))
     {
       /* No... we are still waiting for ACKs.  Drop any received data, but
        * do not yet report TCP_CLOSE in the response.
@@ -357,14 +357,11 @@ static inline int netclose_disconnect(FAR struct socket *psock)
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
   if (psock->s_sndcb)
     {
-      tcp_callback_free(conn, psock->s_sndcb);
       psock->s_sndcb = NULL;
     }
 #endif
 
-  /* There shouldn't be any callbacks registered. */
-
-  DEBUGASSERT(conn && conn->list == NULL);
+  DEBUGASSERT(conn != NULL);
 
   /* Check for the case where the host beat us and disconnected first */