libnvdimm, namespace: make 'resource' attribute only readable by root

For the same reason that /proc/iomem returns 0's for non-root readers
and acpi tables are root-only, make the 'resource' attribute for
namespace devices only readable by root. Otherwise we disclose physical
address information.

Fixes: bf9bccc1 ("libnvdimm: pmem label sets and namespace instantiation")
Cc: <stable@vger.kernel.org>
Reported-by: NDave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/namespace_devs.c

@@ -1620,7 +1620,7 @@ static umode_t namespace_visible(struct kobject *kobj,
 	if (a == &dev_attr_resource.attr) {
 		if (is_namespace_blk(dev))
 			return 0;
-		return a->mode;
+		return 0400;
 	}
 
 	if (is_namespace_pmem(dev) || is_namespace_blk(dev)) {