perf stat: Fix cpu map index in bperf cgroup code

perf stat: Fix cpu map index in bperf cgroup code

The previous cpu map introduced a bug in the bperf cgroup counter.  This
results in a failure when user gives a partial cpu map starting from
non-zero.

  $ sudo ./perf stat -C 1-2 --bpf-counters --for-each-cgroup ^. sleep 1
  libbpf: prog 'on_cgrp_switch': failed to create BPF link for perf_event FD 0:
                                 -9 (Bad file descriptor)
  Failed to attach cgroup program

To get the FD of an evsel, it should use a map index not the CPU number.

Fixes: 3dba364240fe96e8 ("perf cpumap: Switch to using perf_cpu_map API")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: bpf@vger.kernel.org
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220916184132.1161506-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>