cgroup: call cgroup_subsys->bind on cgroup subsys initialization

Currently, we call cgroup_subsys->bind only on unmount, remount, and
when creating a new root on mount. Since the default hierarchy root is
created in cgroup_init, we will not call cgroup_subsys->bind if the
default hierarchy is freshly mounted. As a result, some controllers will
behave incorrectly (most notably, the "memory" controller will not
enable hierarchy support). Fix this by calling cgroup_subsys->bind right
after initializing a cgroup subsystem.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -5040,6 +5040,9 @@ int __init cgroup_init(void)
 			WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
 			WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
 		}
+
+		if (ss->bind)
+			ss->bind(init_css_set.subsys[ssid]);
 	}
 
 	cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);