libperf: Introduce perf_evlist_mmap_ops::idx callback

libperf: Introduce perf_evlist_mmap_ops::idx callback

Add the perf_evlist_mmap_ops::idx callback to be called in
mmap_per_cpu() and mmap_per_thread() with current cpu and thread
indexes.

It's used by current aux code, so perf will use this callback to set the
aux index.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20191007125344.14268-16-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>