blkcg: fix error return path in blkg_create()

In blkg_create(), after lookup of parent fails, the control jumps to
error path with the error code encoded into @blkg.  The error path
doesn't use @blkg for the return value.  It returns ERR_PTR(ret).
Make lookup fail path set @ret instead of @blkg.

Note that the parent lookup is guaranteed to succeed at that point and
the condition check is purely for sanity and triggers WARN when fails.
As such, I don't think it's necessary to mark it for -stable.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NVivek Goyal <vgoyal@redhat.com>

block/blk-cgroup.c

@@ -238,7 +238,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 	if (blkcg_parent(blkcg)) {
 		blkg->parent = __blkg_lookup(blkcg_parent(blkcg), q, false);
 		if (WARN_ON_ONCE(!blkg->parent)) {
-			blkg = ERR_PTR(-EINVAL);
+			ret = -EINVAL;
 			goto err_put_css;
 		}
 		blkg_get(blkg->parent);