thermal: lock the thermal zone when switching governors

Currently, userspace can request a governor change while the governor
itself is running.  Grab the thermal zone lock when changing the
governor to prevent this race.

Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/thermal_core.c

@@ -757,6 +757,7 @@ policy_store(struct device *dev, struct device_attribute *attr,
 	snprintf(name, sizeof(name), "%s", buf);
 
 	mutex_lock(&thermal_governor_lock);
+	mutex_lock(&tz->lock);
 
 	gov = __find_governor(strim(name));
 	if (!gov)
@@ -766,6 +767,7 @@ policy_store(struct device *dev, struct device_attribute *attr,
 	ret = count;
 
 exit:
+	mutex_unlock(&tz->lock);
 	mutex_unlock(&thermal_governor_lock);
 	return ret;
 }