perfcounters: fix refcounting bug

don't kfree in use counters.

Running...

	while true; do perfstat -e 1 -c true; done

...on all cores for a while doesn't seem to be eating ram, and my oops
is gone.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>