perf tools: Add read_all_cgroups() and __cgroup_find()

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 <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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