[Virtio-fs] [virtiofsd] MR opened: passthrough: Fix InodeData reference counting

[virtiofs-bot at sinrega.org]

Previously, the `InodeData` reference counting could increment the
refcount from 0 to 1. See #25. We fix this by using a CAS loop.

Closes: #25

Also, in `forget_one()`, we change the ordering of decrementing the
refcount to `Ordering::Relaxed`, because the refcount itself doesn't
protect any data. Furthermore, the old ordering `Ordering::Release`
doesn't make any sense because there's no side-effect that could
potentially get reordered to after the decrement.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/82