cgroup: Expose some cgroup helpers

cgroup: Expose some cgroup helpers

Expose the following through cgroup.h:

- cgroup_on_dfl()
- cgroup_is_dead()
- cgroup_for_each_live_child()
- cgroup_for_each_live_descendant_pre()
- cgroup_for_each_live_descendant_post()

Until now, these didn't need to be exposed because controllers only cared
about the css hierarchy. The planned sched_ext hierarchical scheduler
support will be based on the default cgroup hierarchy, which is in line
with the existing BPF cgroup support, and thus needs these exposed.

Signed-off-by: Tejun Heo <tj@kernel.org>