sched: fix warning in kernel/sched.c

Impact: fix cpumask conversion bug

this warning:

  kernel/sched.c: In function ‘find_busiest_group’:
  kernel/sched.c:3429: warning: passing argument 1 of ‘__first_cpu’ from incompatible pointer type

shows that we forgot to convert a new patch to the new cpumask APIs.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -3394,7 +3394,7 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 		*imbalance = min_load_per_task;
 		if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP) {
 			cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu =
-					first_cpu(group_leader->cpumask);
+				cpumask_first(sched_group_cpus(group_leader));
 		}
 		return group_min;
 	}