Clear socket write queue and send RST if connection is dropped

Connection can be dropped by calling `ss_conn_drop_guard_exit`
from several places:
- from `tcp_done, when socket is already DEAD
- from `__sk_close_locked` if all data is already sent
- from `ss_linkerror`

In all places socket write queue is already empty, except one
place, when we call `ss_linkerror` for already shutdowned connection.
In this case we should close socket using TCP RST, because we can't
send any more data after connection will be dropped, moreover sequence
numbers of skbs in socket write queue is not correct (they should be
corrected during tls encryption, but appropriate function will never
called for already dropped connection).

Part of #2267

fw/sock.c

@@ -735,9 +735,9 @@ ss_close_not_connected_socket(struct sock *sk)
  * for server sockets.
  */
 static void
-ss_linkerror(struct sock *sk)
+ss_linkerror(struct sock *sk, int flags)
 {
-	ss_do_close(sk, 0);
+	ss_do_close(sk, flags);
 	/*
 	 * In case when ss_do_close is called for TCP_FIN_WAIT2
 	 * tcp_done() is called from tcp_time_wait() and connection
@@ -1142,7 +1142,7 @@ ss_tcp_state_change(struct sock *sk)
 			 */
 			if (!sk->sk_user_data)
 				ss_active_guard_exit(SS_V_ACT_LIVECONN);
-			ss_linkerror(sk);
+			ss_linkerror(sk, 0);
 			ss_active_guard_exit(SS_V_ACT_NEWCONN);
 			return;
 		}
@@ -1195,7 +1195,7 @@ ss_tcp_state_change(struct sock *sk)
 		 */
 		if (!skb_queue_empty(&sk->sk_receive_queue))
 			ss_tcp_process_data(sk);
-		ss_linkerror(sk);
+		ss_linkerror(sk, 0);
 	}
 }
 
@@ -1570,7 +1570,7 @@ ss_tx_action(void)
 		} else if (sk->sk_user_data
 			   && (SS_CONN_TYPE(sk) & Conn_Shutdown)) {
 			if (ss_is_closed_force(&sw))
-				ss_linkerror(sk);
+				ss_linkerror(sk, SS_F_ABORT);
 			bh_unlock_sock(sk);
 			goto dead_sock;
 		}