intel_pstate: Use del_timer_sync in intel_pstate_cpu_stop

Ensure that no timer callback is running since we are about to free
the timer structure.  We cannot guarantee that the call back is called
on the CPU where the timer is running.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/intel_pstate.c

@@ -778,7 +778,7 @@ static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
 
 	pr_info("intel_pstate CPU %d exiting\n", cpu_num);
 
-	del_timer(&all_cpu_data[cpu_num]->timer);
+	del_timer_sync(&all_cpu_data[cpu_num]->timer);
 	intel_pstate_set_pstate(cpu, cpu->pstate.min_pstate);
 	kfree(all_cpu_data[cpu_num]);
 	all_cpu_data[cpu_num] = NULL;