The reason codes are handled in two ways nowadays (quoting Mat Martineau):
1. Sending in the MPTCP option on RST packets when there is no subflow
context available (these use subflow_add_reset_reason() and directly call
a TCP-level send_reset function)
2. The "normal" way via subflow->reset_reason. This will propagate to both
the outgoing reset packet and to a local path manager process via netlink
in mptcp_event_sub_closed()

RFC 8684 defines the skb reset reason behaviour which is not required
even though in some places:

    A host sends a TCP RST in order to close a subflow or reject
    an attempt to open a subflow (MP_JOIN). In order to let the
    receiving host know why a subflow is being closed or rejected,
    the TCP RST packet MAY include the MP_TCPRST option (Figure 15).
    The host MAY use this information to decide, for example, whether
    it tries to re-establish the subflow immediately, later, or never.

Since the commit dc87efdb ("mptcp: add mptcp reset option support")
introduced this feature about three years ago, we can fully use it.
There remains some places where we could insert reason into skb as
we can see in this patch.

Many thanks to Mat and Paolo for help:)
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>