dax: constify the struct device_type usage

Since commit aed65af1 ("drivers: make device_type const"), the driver
core can properly handle constant struct device_type. Move the
dax_mapping_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>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20240219-device_cleanup-dax-v1-1-6b319ee89dc2@marliere.netSigned-off-by: Ira Weiny <ira.weiny@intel.com>

drivers/dax/bus.c

@@ -818,7 +818,7 @@ static const struct attribute_group *dax_mapping_attribute_groups[] = {
 	NULL,
 };
 
-static struct device_type dax_mapping_type = {
+static const struct device_type dax_mapping_type = {
 	.release = dax_mapping_release,
 	.groups = dax_mapping_attribute_groups,
 };