device-dax: Add a 'modalias' attribute to DAX 'bus' devices

Add a 'modalias' attribute to devices under the DAX bus so that userspace
is able to dynamically load modules as needed.

Normally, udev can get the modalias from 'uevent', and that is correctly
set up by the DAX bus. However other tooling such as 'libndctl' for
interacting with drivers/nvdimm/, and 'libdaxctl' for drivers/dax/ can
also use the modalias to dynamically load modules via libkmod lookups.

The 'nd' bus set up by the libnvdimm subsystem exports a modalias
attribute. Imitate this to export the same for the 'dax' bus.

Cc: Dave Hansen <[e-mail]>
Signed-off-by: Vishal Verma <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/dax/bus.c

···
 }
 static DEVICE_ATTR_RO(target_node);
 
+static ssize_t modalias_show(struct device *dev, struct device_attribute *attr,
+		char *buf)
+{
+	/*
+	 * We only ever expect to handle device-dax instances, i.e. the
+	 * @type argument to MODULE_ALIAS_DAX_DEVICE() is always zero
+	 */
+	return sprintf(buf, DAX_DEVICE_MODALIAS_FMT "\n", 0);
+}
+static DEVICE_ATTR_RO(modalias);
+
 static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, int n)
 {
 	struct device *dev = container_of(kobj, struct device, kobj);
···
 }
 
 static struct attribute *dev_dax_attributes[] = {
+	&dev_attr_modalias.attr,
 	&dev_attr_size.attr,
 	&dev_attr_target_node.attr,
 	NULL,