cgroup: bpf: enable bpf programs to integrate with rstat

Enable bpf programs to make use of rstat to collect cgroup hierarchical
stats efficiently:
- Add cgroup_rstat_updated() kfunc, for bpf progs that collect stats.
- Add cgroup_rstat_flush() sleepable kfunc, for bpf progs that read stats.
- Add an empty bpf_rstat_flush() hook that is called during rstat
  flushing, for bpf progs that flush stats to attach to. Attaching a bpf
  prog to this hook effectively registers it as a flush callback.

Signed-off-by: Yosry Ahmed <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Hao Luo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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