perf stat: Fix metric-only aggregation index

Aggregation index was being computed using the evsel's cpumap which
may have a different (typically the same or fewer) entries.

Before:
```
$ perf stat --metric-only -A -M memory_bandwidth_total -a sleep 1

 Performance counter stats for 'system wide':

       MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total
CPU0                            12.8                           0.0                          12.9                          12.7                           0.0                          12.6
CPU1

       1.007806367 seconds time elapsed
```

After:
```
$ perf stat --metric-only -A -M memory_bandwidth_total -a sleep 1

 Performance counter stats for 'system wide':

       MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total MB/s  memory_bandwidth_total
CPU0                            15.4                           0.0                          15.3                          15.0                           0.0                          14.9
CPU18                            0.0                           0.0                          13.5                           5.2                           0.0                          11.9

       1.007858736 seconds time elapsed
```

Signed-off-by: Ian Rogers <irogers@google.com>                                  |
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Kaige Ye <ye@kaige.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240221070754.4163916-3-irogers@google.com