block: rnull: add missing MODULE_DESCRIPTION

Add the missing description to fix the following warning:

WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/block/rnull_mod.o

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20241130094521.193924-1-fujita.tomonori@gmail.com


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