mptcp: be careful on subflows shutdown

When the workqueue disposes of the msk, the subflows can still
receive some data from the peer after __mptcp_close_ssk()
completes.

The above could trigger a race between the msk receive path and the
msk destruction. Acquiring the mptcp_data_lock() in __mptcp_destroy_sock()
will not save the day: the rx path could be reached even after msk
destruction completes.

Instead use the subflow 'disposable' flag to prevent entering
the msk receive path after __mptcp_close_ssk().

Fixes: e16163b6 ("mptcp: refactor shutdown and close")
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>