ntco::IoRing: data race when accessing ntcs::Event::status field

[smtrfnv]

Describe the bug
When IoRingDevice does not support event cancellation by handle it is possible that there will be a data race during event cancellation: multiple threads would access status of an event and try to write there. Because of that ntcf_system.t tests can fail sometimes.
Also, ntcs::Event currently is common for multiple drivers, probably we need to implement something specific for IoRing.