cgroup: don't online subsystems before cgroup_name/path() are operational

While refactoring cgroup creation, a5bca2152036 ("cgroup: factor out
cgroup_create() out of cgroup_mkdir()") incorrectly onlined subsystems
before the new cgroup is associated with it kernfs_node.  This is fine
for cgroup proper but cgroup_name/path() depend on the associated
kernfs_node and if a subsystem makes the new cgroup_subsys_state
visible, which they're allowed to after onlining, it can lead to NULL
dereference.

The current code performs cgroup creation and subsystem onlining in
cgroup_create() and cgroup_mkdir() makes the cgroup and subsystems
visible afterwards.  There's no reason to online the subsystems early
and we can simply drop cgroup_apply_control_enable() call from
cgroup_create() so that the subsystems are onlined and made visible at
the same time.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: a5bca2152036 ("cgroup: factor out cgroup_create() out of cgroup_mkdir()") 
Cc: stable@vger.kernel.org # v4.6+

path: root/net/core/net-sysfs.h

#ifndef __NET_SYSFS_H__
#define __NET_SYSFS_H__

int __init netdev_kobject_init(void);
int netdev_register_kobject(struct net_device *);
void netdev_unregister_kobject(struct net_device *);
int net_rx_queue_update_kobjects(struct net_device *, int old_num, int new_num);
int netdev_queue_update_kobjects(struct net_device *net,
				 int old_num, int new_num);

#endif