cgroup: Add root slice to cgroup realization queue

cgroup: Add root slice to cgroup realization queue

When we're disabling controller on a direct child of root cgroup, we
forgot to add root slice into cgroup realization queue, which prevented
proper disabling of the controller (on unified hierarchy).

The mechanism relying on "bounce from bottom and propagate up" in
unit_create_cgroup doesn't work on unified hierarchy (leaves needn't be
enabled). Drop it as we rely on the ancestors to be queued -- that's now
intentional but was artifact of combining the two patches:

cb5e3bc37d ("cgroup: Don't explicitly check for member in UNIT_BEFORE") v240~78
65f6b6bdcb ("core: fix re-realization of cgroup siblings") v245-rc1~153^2

Fixes: #14917