blk-cgroup: Fix memleak on error path

If new_blkg allocation raced with blk_policy change and
blkg_lookup_check fails, new_blkg is leaked.
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NGabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -663,6 +663,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		blkg = blkg_lookup_check(pos, pol, q);
 		if (IS_ERR(blkg)) {
 			ret = PTR_ERR(blkg);
+			blkg_free(new_blkg);
 			goto fail_unlock;
 		}