rnull: move driver to separate directory

The rust null block driver is about to gain some additional modules. Rather
than pollute the current directory, move the driver to a subdirectory.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-12-b5212cc89b98@kernel.org


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