mptcp: don't orphan ssk in mptcp_close()

All of the subflows of a msk will be orphaned in mptcp_close(), which
means the subflows are in DEAD state. After then, DATA_FIN will be sent,
and the other side will response with a DATA_ACK for this DATA_FIN.

However, if the other side still has pending data, the data that received
on these subflows will not be passed to the msk, as they are DEAD and
subflow_data_ready() will not be called in tcp_data_ready(). Therefore,
these data can't be acked, and they will be retransmitted again and again,
until timeout.

Fix this by setting ssk->sk_socket and ssk->sk_wq to 'NULL', instead of
orphaning the subflows in __mptcp_close(), as Paolo suggested.

Fixes: e16163b ("mptcp: refactor shutdown and close")
Reviewed-by: Biao Jiang <benbjiang@tencent.com>
Reviewed-by: Mengen Sun <mengensun@tencent.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Reviewed-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/protocol.c

@@ -2354,12 +2354,7 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
  goto out;
  }
 
- /* if we are invoked by the msk cleanup code, the subflow is
- * already orphaned
- */
- if (ssk->sk_socket)
- sock_orphan(ssk);
-
+ sock_orphan(ssk);
  subflow->disposable = 1;
 
  /* if ssk hit tcp_done(), tcp_cleanup_ulp() cleared the related ops
@@ -2940,7 +2935,11 @@ bool __mptcp_close(struct sock *sk, long timeout)
  if (ssk == msk->first)
  subflow->fail_tout = 0;
 
- sock_orphan(ssk);
+ /* detach from the parent socket, but allow data_ready to
+ * push incoming data into the mptcp stack, to properly ack it
+ */
+ ssk->sk_socket = NULL;
+ ssk->sk_wq = NULL;
  unlock_sock_fast(ssk, slow);
  }
  sock_orphan(sk);