Move cgroup_lock_live_group() invocation upwards to right below
cgroup_tree_mutex in cgroup_subtree_control_write().  This is to help
the planned locking simplification.

This doesn't make any userland-visible behavioral changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>