MEDIUM: counters: change the fill_stats() API to pass the module and extra_counters

MEDIUM: counters: change the fill_stats() API to pass the module and extra_counters

We'll soon need to iterate over thread groups in the fill_stats() functions,
so let's first pass the extra_counters and stats_module pointers to the
fill_stats functions. They now call EXTRA_COUNTERS_GET() themselves with
these elements in order to retrieve the required pointer. Nothing else
changed, and it's getting even a bit more transparent for callers.

This doesn't change anything visible however.