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>

kernel/perf_counter.c

@@ -1934,7 +1934,8 @@ __perf_counter_exit_task(struct task_struct *child,
 		}
 	}
 
-	kfree(child_counter);
+	if (!child_counter->filp || !atomic_long_read(&child_counter->filp->f_count))
+		kfree(child_counter);
 }
 
 /*