Is your feature request related to a problem? Please describe.
It would be useful for plugins to be able to identify the reason of a subflow closure in the relevant callback, for example to detect if a subflow was closed for a network error.
The pm_event_attrs->error field looks useful for this purpose.

Describe the solution you'd like
The mptcpd_plugin_subflow_closed() be called with additional argument error.
Or maybe pass the entire pm_event_attrs struct.