rbd: add __init/__exit annotations

Add __init/__exit annotation to init/cleanup helpers
which are only called once in the module.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>