blk-iocost: Remove unnecessary (void*) conversions

The key pointer is void and hence does not need an explicit cast.

Signed-off-by: Li zeming <zeming@nfschina.com>
Link: https://lore.kernel.org/r/20220919012825.2936-1-zeming@nfschina.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>