rust: configfs: re-export `configfs_attrs` from `configfs` module

rust: configfs: re-export `configfs_attrs` from `configfs` module

Re-export `configfs_attrs` from `configfs` module, so that users can import
the macro from the `configfs` module rather than the root of the `kernel`
crate.

Also update users to import from the new path.

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-v7-6-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>