rbd: add 'snap_id' sysfs rbd device attribute

Export snap id in sysfs, so tools like multipathd can use it in a uuid.
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Documentation/ABI/testing/sysfs-bus-rbd

@@ -102,6 +102,10 @@ current_snap
 
 	The current snapshot for which the device is mapped.
 
+snap_id
+
+	The current snapshot's id.  (August 2016, since 4.9.)
+
 parent
 
 	Information identifying the chain of parent images in a layered rbd

drivers/block/rbd.c

@@ -4669,6 +4669,14 @@ static ssize_t rbd_snap_show(struct device *dev,
 	return sprintf(buf, "%s\n", rbd_dev->spec->snap_name);
 }
 
+static ssize_t rbd_snap_id_show(struct device *dev,
+				struct device_attribute *attr, char *buf)
+{
+	struct rbd_device *rbd_dev = dev_to_rbd_dev(dev);
+
+	return sprintf(buf, "%llu\n", rbd_dev->spec->snap_id);
+}
+
 /*
  * For a v2 image, shows the chain of parent images, separated by empty
  * lines.  For v1 images or if there is no parent, shows "(no parent
@@ -4730,6 +4738,7 @@ static DEVICE_ATTR(name, S_IRUGO, rbd_name_show, NULL);
 static DEVICE_ATTR(image_id, S_IRUGO, rbd_image_id_show, NULL);
 static DEVICE_ATTR(refresh, S_IWUSR, NULL, rbd_image_refresh);
 static DEVICE_ATTR(current_snap, S_IRUGO, rbd_snap_show, NULL);
+static DEVICE_ATTR(snap_id, S_IRUGO, rbd_snap_id_show, NULL);
 static DEVICE_ATTR(parent, S_IRUGO, rbd_parent_show, NULL);
 
 static struct attribute *rbd_attrs[] = {
@@ -4745,6 +4754,7 @@ static struct attribute *rbd_attrs[] = {
 	&dev_attr_name.attr,
 	&dev_attr_image_id.attr,
 	&dev_attr_current_snap.attr,
+	&dev_attr_snap_id.attr,
 	&dev_attr_parent.attr,
 	&dev_attr_refresh.attr,
 	NULL