block: rnull: Initialize the module in place

Using `InPlaceModule` avoids an allocation and an indirection.

Signed-off-by: Benoît du Garreau <benoit@dugarreau.fr>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20241204-rnull_in_place-v1-1-efe3eafac9fb@dugarreau.fr


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