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 <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>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-xen-v1-2-c2f5fe89ed95@marliere.net
Signed-off-by: Juergen Gross <jgross@suse.com>

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,
 };