topology/sysfs: Hide PPIN on systems that do not support it.

Systems that do not support a Protected Processor Identification Number
currently report:

	# cat /sys/devices/system/cpu/cpu0/topology/ppin
	0x0

which is confusing/wrong.

Add a ".is_visible" function to suppress inclusion of the ppin file.

Fixes: ab28e944 ("topology/sysfs: Add PPIN in sysfs under cpu topology")
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20220406220150.63855-1-tony.luck@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/topology.c

@@ -152,9 +152,21 @@ static struct attribute *default_attrs[] = {
 	NULL
 };
 
+static umode_t topology_is_visible(struct kobject *kobj,
+				   struct attribute *attr, int unused)
+{
+	struct device *dev = kobj_to_dev(kobj);
+
+	if (attr == &dev_attr_ppin.attr && !topology_ppin(dev->id))
+		return 0;
+
+	return attr->mode;
+}
+
 static const struct attribute_group topology_attr_group = {
 	.attrs = default_attrs,
 	.bin_attrs = bin_attrs,
+	.is_visible = topology_is_visible,
 	.name = "topology"
 };