The read_all_cgroups() is to build a tree of cgroups in the system and
users can look up a cgroup using __cgroup_find().

Committer notes:

Had to do this to cover that #else block:

  -static inline u64 __read_cgroup_id(const char *path) { return -1ULL; }
  +static inline u64 __read_cgroup_id(const char *path __maybe_unused) { return -1ULL; }
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20230906174903.346486-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>