block: remove unneeded parenthesis from blk-sysfs

Align to common code conventions.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/20210511155319.1885277-1-mgurtovoy@nvidia.com


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