tcp: Flush socket before checking for more data in active close state

Otherwise, if all the pending data is acknowledged:

- tcp_update_seqack_from_tap() updates the current tap-side ACK
  sequence (conn->seq_ack_from_tap)

- next, we compare the sequence we sent (conn->seq_to_tap) to the
  ACK sequence (conn->seq_ack_from_tap) in tcp_data_from_sock() to
  understand if there's more data we can send.

  If they match, we conclude that we haven't sent any of that data,
  and keep re-sending it.

We need, instead, to flush the socket (drop acknowledged data) before
calling tcp_update_seqack_from_tap(), so that once we update
conn->seq_ack_from_tap, we can be sure that all data until there is
gone from the socket.

Link: https://bugs.passt.top/show_bug.cgi?id=114
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Fixes: 30f1e082c3 ("tcp: Keep updating window and checking for socket data after FIN from guest")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit ebdd46367c)

tcp.c

@@ -2047,6 +2047,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 
 	/* Established connections not accepting data from tap */
 	if (conn->events & TAP_FIN_RCVD) {
+		tcp_sock_consume(conn, ntohl(th->ack_seq));
 		tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
 		tcp_tap_window_update(conn, ntohs(th->window));
 		tcp_data_from_sock(c, conn);