perf_counter: round-robin per-CPU counters too

This used to be unstable when we had the rq->lock dependencies,
but now that they are that of the past we can turn on percpu
counter RR too.

[ Impact: handle counter over-commit for per-CPU counters too ]

LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>