sched_ext: Add a cgroup scheduler which uses flattened hierarchy

This patch adds scx_flatcg example scheduler which implements hierarchical
weight-based cgroup CPU control by flattening the cgroup hierarchy into a
single layer by compounding the active weight share at each level.

This flattening of hierarchy can bring a substantial performance gain when
the cgroup hierarchy is nested multiple levels. in a simple benchmark using
wrk[8] on apache serving a CGI script calculating sha1sum of a small file,
it outperforms CFS by ~3% with CPU controller disabled and by ~10% with two
apache instances competing with 2:1 weight ratio nested four level deep.

However, the gain comes at the cost of not being able to properly handle
thundering herd of cgroups. For example, if many cgroups which are nested
behind a low priority parent cgroup wake up around the same time, they may
be able to consume more CPU cycles than they are entitled to. In many use
cases, this isn't a real concern especially given the performance gain.
Also, there are ways to mitigate the problem further by e.g. introducing an
extra scheduling layer on cgroup delegation boundaries.

v5: - Updated to specify SCX_OPS_HAS_CGROUP_WEIGHT instead of
      SCX_OPS_KNOB_CGROUP_WEIGHT.

v4: - Revert reference counted kptr for cgv_node as the change caused easily
      reproducible stalls.

v3: - Updated to reflect the core API changes including ops.init/exit_task()
      and direct dispatch from ops.select_cpu(). Fixes and improvements
      including additional statistics.

    - Use reference counted kptr for cgv_node instead of xchg'ing against
      stash location.

    - Dropped '-p' option.

v2: - Use SCX_BUG[_ON]() to simplify error handling.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>

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