rnull: enable configuration via `configfs`

Allow rust null block devices to be configured and instantiated via
`configfs`.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-13-b5212cc89b98@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>