blk-throttle: only enable blk-stat when BLK_DEV_THROTTLING_LOW

blk-throttle: only enable blk-stat when BLK_DEV_THROTTLING_LOW

blk_throtl_register() will unconditionally enable blk-stat for gendisk
when register, even when we have no BLK_DEV_THROTTLING_LOW config.

Since the kernel always has only BLK_DEV_THROTTLING config and the
BLK_DEV_THROTTLING_LOW config is still in EXPERIMENTAL state, we can
just skip blk-stat when !BLK_DEV_THROTTLING_LOW.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230413062805.2081970-2-chengming.zhou@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>