devfreq:governor:Save void *data in the governor userspace

The member void *data in the structure devfreq can be overwrite by governor_userspace. For example:
1. The device driver assigned the devfreq governor to simple_ondemand by the function devfreq_add_device() and init the devfreq member void *data to a pointer of a static structure devfreq_simple_ondemand_data by the function devfreq_add_device().
2. The user changed the devfreq governor to userspace by the command "echo userspace > /sys/class/devfreq/.../governor".
3. The governor userspace alloced a dynamic memory for the struct userspace_data and assigend the member void *data of devfreq to this memory by the function userspace_init().
4. The user changed the devfreq governor back to simple_ondemand by the command "echo simple_ondemand > /sys/class/devfreq/.../governor".
5. The governor userspace exited and assigned the member void *data in the structure devfreq to NULL by the function userspace_exit().
6. The governor simple_ondemand fetched the static information of devfreq_simple_ondemand_data in the function devfreq_simple_ondemand_func() but the member void *data of devfreq was assigned to NULL by the function userspace_exit().
7. The information of upthreshold and downdifferential is lost and the governor simple_ondemand can't work correctly.

The member void *data in the structure devfreq is designed for a static pointer used in a governor and inited by the function devfreq_add_device(). So if a governor want to use void *data to do some other things, it must save void *data in the init() function and restore void *data in the exit() function.

---
 drivers/devfreq/governor_userspace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

index ab9db7adb3ad..dbbb448dcbcf 100644
@@ -17,6 +17,7 @@
 struct userspace_data {
 	unsigned long user_frequency;
 	bool valid;
+	void *saved_data;
 };
 
 static int devfreq_userspace_func(struct devfreq *df, unsigned long *freq)

@@ -91,6 +92,7 @@ static int userspace_init(struct devfreq *devfreq)
 		goto out;
 	}
 	data->valid = false;
+	data->saved_data = devfreq->data;
 	devfreq->data = data;
 
 	err = sysfs_create_group(&devfreq->dev.kobj, &dev_attr_group);

@@ -100,6 +102,8 @@ static int userspace_init(struct devfreq *devfreq)
 
 static void userspace_exit(struct devfreq *devfreq)
 {
+	struct userspace_data *data = devfreq->data;
+	void *saved_data = data->saved_data;
 	/*
 	 * Remove the sysfs entry, unless this is being called after
 	 * device_del(), which should have done this already via kobject_del().

@@ -108,7 +112,7 @@ static void userspace_exit(struct devfreq *devfreq)
 		sysfs_remove_group(&devfreq->dev.kobj, &dev_attr_group);
 
 	kfree(devfreq->data);
-	devfreq->data = NULL;
+	devfreq->data = saved_data;
 }
 
 static int devfreq_userspace_handler(struct devfreq *devfreq,

2.29.0