nvdimm: make nvdimm_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the nvdimm_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>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-nvdimm-v1-1-77ae19fa3e3b@marliere.netSigned-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/nvdimm/bus.c

@@ -271,7 +271,7 @@ EXPORT_SYMBOL_GPL(nvdimm_clear_poison);
 
 static int nvdimm_bus_match(struct device *dev, struct device_driver *drv);
 
-static struct bus_type nvdimm_bus_type = {
+static const struct bus_type nvdimm_bus_type = {
 	.name = "nd",
 	.uevent = nvdimm_bus_uevent,
 	.match = nvdimm_bus_match,