perf tools: Add read_cgroup_id() function

perf tools: Add read_cgroup_id() function

The read_cgroup_id() is to read a cgroup id from a file handle using
name_to_handle_at(2) for the given cgroup.  It'll be used by bperf
cgroup stat later.

Committer notes:

  -int read_cgroup_id(struct cgroup *cgrp)
  +static inline int read_cgroup_id(struct cgroup *cgrp __maybe_unused)

To fix the build when HAVE_FILE_HANDLE is not defined.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210625071826.608504-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>