cgroup: separate out cgroup_calc_child_subsys_mask() from cgroup_refresh_child_subsys_mask()
cgroup: separate out cgroup_calc_child_subsys_mask() from cgroup_refresh_child_subsys_mask()

cgroup_refresh_child_subsys_mask() calculates and updates the
effective @cgrp->child_subsys_maks according to the current
@cgrp->subtree_control.  Separate out the calculation part into
cgroup_calc_child_subsys_mask().  This will be used to fix a bug in
the async css offline wait logic.

Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Acked-by: Zefan Li <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs