workqueue: make wq_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the wq_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-and-reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -6692,7 +6692,7 @@ static struct device_attribute wq_sysfs_unbound_attrs[] = {
 	__ATTR_NULL,
 };
 
-static struct bus_type wq_subsys = {
+static const struct bus_type wq_subsys = {
 	.name				= "workqueue",
 	.dev_groups			= wq_sysfs_groups,
 };