MEDIUM: counters: factor out smp_fetch_sc*_trackers

MEDIUM: counters: factor out smp_fetch_sc*_trackers

smp_fetch_sc0_trackers, smp_fetch_sc1_trackers and smp_fetch_sc2_trackers
were merged into a single function which relies on the fetch name to decide
what to return.

This is also a bug fix for this feature which has never worked till its bogus
introduction by commit "2406db4 MEDIUM: counters: add sc1_trackers/sc2_trackers"
(1.5-dev10).

Instead of returning the value in the sample, it was returned as the fetch
result!

There is no need to backport this fix anyway since it's 1.5-specific and
nobody uses the feature.