dax: Add numa_node to the default device-dax attributes

It is confusing that device-dax instances publish a 'target_node'
attribute, but not a 'numa_node'. The 'numa_node' information is
available elsewhere in the sysfs device hierarchy, but it is not obvious
and not reliable from one device-dax instance-type (e.g. child devices
of nvdimm namespaces) to the next (e.g. 'hmem' devices defined by EFI
Specific Purpose Memory and the ACPI HMAT).

Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/157309906102.1582359.4262088001244476001.stgit@dwillia2-desk3.amr.corp.intel.com

drivers/dax/bus.c

@@ -322,6 +322,13 @@ static ssize_t modalias_show(struct device *dev, struct device_attribute *attr,
 }
 static DEVICE_ATTR_RO(modalias);
 
+static ssize_t numa_node_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", dev_to_node(dev));
+}
+static DEVICE_ATTR_RO(numa_node);
+
 static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, int n)
 {
 	struct device *dev = container_of(kobj, struct device, kobj);
@@ -329,6 +336,8 @@ static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, int n)
 
 	if (a == &dev_attr_target_node.attr && dev_dax_target_node(dev_dax) < 0)
 		return 0;
+	if (a == &dev_attr_numa_node.attr && !IS_ENABLED(CONFIG_NUMA))
+		return 0;
 	return a->mode;
 }
 
@@ -337,6 +346,7 @@ static struct attribute *dev_dax_attributes[] = {
 	&dev_attr_size.attr,
 	&dev_attr_target_node.attr,
 	&dev_attr_resource.attr,
+	&dev_attr_numa_node.attr,
 	NULL,
 };