blk-cgroup: remove blk_queue_root_blkg

blk-cgroup: remove blk_queue_root_blkg

[ Upstream commit 928f6f00a91ecbef6cb1fe59474831ceaf205290 ]

Just open code it in the only caller and drop the unused !BLK_CGROUP
stub.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Herrmann <aherrmann@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220921180501.1539876-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 813e693023ba ("blk-iolatency: Fix memory leak on add_disk() failures")
Signed-off-by: Sasha Levin <sashal@kernel.org>