blk-iocost: fix queue stats accounting

The q->stats->accounting is not only used by iocost, but iocost only
increase this counter, never decrease it. So queue stats accounting
will always enabled after using iocost once.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230804070609.31623-1-chengming.zhou@linux.dev


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