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

block/blk-throttle.c

@@ -2443,11 +2443,12 @@ void blk_throtl_register(struct gendisk *disk)
 #ifndef CONFIG_BLK_DEV_THROTTLING_LOW
 	/* if no low limit, use previous default */
 	td->throtl_slice = DFL_THROTL_SLICE_HD;
-#endif
 
+#else
 	td->track_bio_latency = !queue_is_mq(q);
 	if (!td->track_bio_latency)
 		blk_stat_enable_accounting(q);
+#endif
 }
 
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW