blk-cgroup: fix a hd_struct leak in blkcg_fill_root_iostats

disk_get_part needs to be paired with a disk_put_part.

Cc: stable@vger.kernel.org
Fixes: ef45fe47 ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>