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>

block/blk-cgroup.c

@@ -849,6 +849,7 @@ static void blkcg_fill_root_iostats(void)
 			blkg_iostat_set(&blkg->iostat.cur, &tmp);
 			u64_stats_update_end(&blkg->iostat.sync);
 		}
+		disk_put_part(part);
 	}
 }