perf auxtrace: Do not mix up mmap idx

perf auxtrace: Do not mix up mmap idx

The idx is with respect to evlist not evsel. That hasn't mattered because
they are the same at present. Prepare for that not being the case, which it
won't be when sideband tracking events are allowed on all CPUs even when
auxtrace is limited to selected CPUs.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20220506122601.367589-6-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>