xen: balloon: make balloon_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the balloon_subsys 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 <[email protected]>
Suggested-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Ricardo B. Marliere <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Juergen Gross <[email protected]>

Author: rbmarliere

drivers/xen/xen-balloon.c

@@ -237,7 +237,7 @@ static const struct attribute_group *balloon_groups[] = {
 	NULL
 };
 
-static struct bus_type balloon_subsys = {
+static const struct bus_type balloon_subsys = {
 	.name = BALLOON_CLASS_NAME,
 	.dev_name = BALLOON_CLASS_NAME,
 };