After e0e8bf083 ("spa: libcamera: source: create eventfd before starting camera"),
things are still not entirely correct. This change ensures that if starting
the camera fails, then the runtime state, most importantly the ring buffer
of completed requests is restored to its initial state.
Furthermore, it is also ensured that `impl::active` can never be observed
by the data thread while it is being changed, which is achieved by setting
it before/after adding/removing the event source.
3e28f3e859