sched/psi: Save percpu memory when !psi_cgroups_enabled

We won't use cgroup psi_group when !psi_cgroups_enabled, so don't
bother to alloc percpu memory and init for it.

Also don't need to migrate task PSI stats between cgroups in
cgroup_move_task().

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20220825164111.29534-4-zhouchengming@bytedance.com