selftests/bpf: extend cgroup helpers

This patch extends bpf selft cgroup_helpers [ID] n various ways:
- Add enable_controllers() that allows tests to enable all or a
  subset of controllers for a specific cgroup.
- Add join_cgroup_parent(). The cgroup workdir is based on the pid,
  therefore a spawned child cannot join the same cgroup hierarchy of the
  test through join_cgroup(). join_cgroup_parent() is used in child
  processes to join a cgroup under the parent's workdir.
- Add write_cgroup_file() and write_cgroup_file_parent() (similar to
  join_cgroup_parent() above).
- Add get_root_cgroup() for tests that need to do checks on root cgroup.
- Distinguish relative and absolute cgroup paths in function arguments.
  Now relative paths are called relative_path, and absolute paths are
  called cgroup_path.

Signed-off-by: Yosry Ahmed <[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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