vfio: mdev: make mdev_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the mdev_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
Link: https://lore.kernel.org/r/20240208-bus_cleanup-vfio-v1-1-ed5da3019949@marliere.netSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/mdev/mdev_driver.c

@@ -40,7 +40,7 @@ static int mdev_match(struct device *dev, struct device_driver *drv)
 	return 0;
 }
 
-struct bus_type mdev_bus_type = {
+const struct bus_type mdev_bus_type = {
 	.name		= "mdev",
 	.probe		= mdev_probe,
 	.remove		= mdev_remove,

drivers/vfio/mdev/mdev_private.h

@@ -13,7 +13,7 @@
 int  mdev_bus_register(void);
 void mdev_bus_unregister(void);
 
-extern struct bus_type mdev_bus_type;
+extern const struct bus_type mdev_bus_type;
 extern const struct attribute_group *mdev_device_groups[];
 
 #define to_mdev_type_attr(_attr)	\