cgroup: Changes for v6.6

* Per-cpu cpu usage stats are now tracked. This currently isn't printed out
  in the cgroupfs interface and can only be accessed through e.g. BPF.
  Should decide on a not-too-ugly way to show per-cpu stats in cgroupfs.

* cpuset received some cleanups and prepatory patches for the pending
  cpus.exclusive patchset which will allow cpuset partitions to be created
  below non-partition parents, which should ease the management of partition
  cpusets.

* A lot of code and documentation cleanup patches.

* tools/testing/selftests/cgroup/test_cpuset.c is added. This causes trivial
  conflicts in .gitignore and Makefile under the directory against
  fe3b1bf19bdf ("selftests: cgroup: add test_zswap program"). They can be
  resolved by keeping lines from both branches.
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