blk-cgroup: store a gendisk to throttle in struct task_struct

blk-cgroup: store a gendisk to throttle in struct task_struct

Switch from a request_queue pointer and reference to a gendisk once
for the throttle information in struct task_struct.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Herrmann <aherrmann@suse.de>
Link: https://lore.kernel.org/r/20230203150400.3199230-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>