MINOR: counters: provide a generic function to retrieve a stkctr for sc* and src.

MINOR: counters: provide a generic function to retrieve a stkctr for sc* and src.

This function aims at simplifying the prefetching of the table and entry
when using any of the session counters fetches. The principle is that the
src_* variant produces a stkctr that is used instead of the one from the
session. That way we can call the same function from all session counter
fetch functions and always have a single function to support sc[0-9]_/src_.