null_blk: Fix the WARNING: modpost: missing MODULE_DESCRIPTION()

[ Upstream commit 9e6727f8 ]

No functional changes intended.

Fixes: f2298c04

 ("null_blk: multi queue aware block test driver")
Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20240506075538.6064-1-yanjun.zhu@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>