xen/xenbus: move to_xenbus_device() to use container_of_const()

The driver core is changing to pass some pointers as const, so move
to_xenbus_device() to use container_of_const() to handle this change.

to_xenbus_device() now properly keeps the const-ness of the pointer passed
into it, while as before it could be lost.

Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230111113018.459199-15-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/xen/xenbus.h

@@ -96,10 +96,7 @@ struct xenbus_device {
 	unsigned int spurious_threshold;
 };
 
-static inline struct xenbus_device *to_xenbus_device(struct device *dev)
-{
-	return container_of(dev, struct xenbus_device, dev);
-}
+#define to_xenbus_device(__dev)	container_of_const(__dev, struct xenbus_device, dev)
 
 struct xenbus_device_id
 {