cpufreq: only stop ondemand governor if already started

On CPUFREQ_GOV_STOP in cpufreq_governor_dbs, shortcut to
return success if the governor is already stopped.

Avoid executing dbs_timer_exit, to prevent tripping an assertion
within a call to kill_timer on a timer that has not been prepared
with init_timer, if the CPUFREQ_GOV_START case has not
run beforehand.

kill_timer validates timer state:
 * itself, via BUG_ON(this_cpu(timers).running == timer);
 * within active_timer, ASSERTing timer->status is within bounds;
 * within list_del, which ASSERTs timer inactive list membership.

Patch is synonymous to an OpenXT patch produced at Citrix prior to
June 2014.

Signed-off-by: Christopher Clark <christopher.clark6@baesystems.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/cpufreq/cpufreq_ondemand.c

@@ -273,6 +273,10 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy, unsigned int event)
         break;
 
     case CPUFREQ_GOV_STOP:
+        if ( !this_dbs_info->enable )
+            /* Already not enabled */
+            break;
+
         dbs_timer_exit(this_dbs_info);
         dbs_enable--;