Revert "blk-cgroup: pin the gendisk in struct blkcg_gq"

This reverts commit 84d7d462.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230214183308.1658775-6-hch@lst.de


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