iocost_monitor: fix kernel queue kobj changes

When I use iocost_monitor on nvme0n1, this error shows up:
"Could not find ioc for nvme0n1"

There is no kobj in struct queue in recent kernel, it seems that the commit
2bd85221 ("block: untangle request_queue refcounting from sysfs")
move the queue kobj to struct gendisk.

Fix it by using mq_kobj which is at the same level with queue kobj.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Link: https://lore.kernel.org/r/20230804065039.8885-1-chengming.zhou@linux.dev


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