cgroup: Move populated counters to cgroup_subsys_state

Later patches replace the cgroup-level finish_destroy_work deferral added
by 93618edf7538 ("cgroup: Defer css percpu_ref kill on rmdir until cgroup
is depopulated") with a per-subsys-css deferral. That needs each subsystem
css to track its own populated count. Move the populated counters from
cgroup onto cgroup_subsys_state. cgroup->self is itself a
cgroup_subsys_state and self.parent walks the same chain as cgroup_parent(),
so cgroup_update_populated() generalizes to a single css_update_populated()
taking a css. The cgroup-side bookkeeping runs only when the walk started
from a self css.

Keep nr_populated_{domain,threaded}_children on cgroup. Both sum to
self.nr_populated_children, but staying as dedicated fields to allow readers
like cgroup_can_be_thread_root() unlocked access.

css_set_update_populated() also walks the per-subsys-css chain so each
subsystem css's hierarchical populated count is maintained. No reader
consumes those counts yet.

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

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