[Virtio-fs] [virtiofsd] MR opened: vhost_user_backend: Skip locking if we have exclusive reference

[virtiofs-bot at sinrega.org]

If we have an exclusive reference to the
`Mutex<VhostUserFsThread<F>>`, then we don't need to lock it.
Instead we can use `.get_mut()`.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/83