cgroup: get the wrong css for css_alloc() during cgroup_init_subsys()

css_alloc() needs the parent css, while cgroup_css() gets current
cgropu's css. So we are getting the wrong css during
cgroup_init_subsys().

Fortunately, cgrp_dfl_root.cgrp's css is not set yet, so the value we
pass to css_alloc() is NULL anyway.

Let's pass NULL directly during init, since we know there is no parent
yet.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -5711,7 +5711,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 
 	/* Create the root cgroup state for this subsystem */
 	ss->root = &cgrp_dfl_root;
-	css = ss->css_alloc(cgroup_css(&cgrp_dfl_root.cgrp, ss));
+	css = ss->css_alloc(NULL);
 	/* We don't handle early failures gracefully */
 	BUG_ON(IS_ERR(css));
 	init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);