rbd: wire up is_visible() sysfs callback for rbd bus

In preparation for single-major device number allocation scheme, wire
up attribute_group::is_visible() callback for rbd bus.  This allows us
to make the new single-major attributes conditional.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -408,7 +408,18 @@ static struct attribute *rbd_bus_attrs[] = {
 	&bus_attr_remove.attr,
 	NULL,
 };
-ATTRIBUTE_GROUPS(rbd_bus);
+
+static umode_t rbd_bus_is_visible(struct kobject *kobj,
+				  struct attribute *attr, int index)
+{
+	return attr->mode;
+}
+
+static const struct attribute_group rbd_bus_group = {
+	.attrs = rbd_bus_attrs,
+	.is_visible = rbd_bus_is_visible,
+};
+__ATTRIBUTE_GROUPS(rbd_bus);
 
 static struct bus_type rbd_bus_type = {
 	.name		= "rbd",