rbd: add 'cluster_fsid' sysfs rbd device attribute

Export the cluster fsid, so tools like udev and multipath-tools can use
it for part of the uuid.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/Documentation/ABI/testing/sysfs-bus-rbd b/Documentation/ABI/testing/sysfs-bus-rbd
index 273e27f..9a655f3 100644
--- a/Documentation/ABI/testing/sysfs-bus-rbd
+++ b/Documentation/ABI/testing/sysfs-bus-rbd
@@ -53,6 +53,10 @@
 
 	The ceph unique client id that was assigned for this specific session.
 
+cluster_fsid
+
+	The ceph cluster UUID.  (August 2016, since 4.9.)
+
 features
 
 	A hexadecimal encoding of the feature bits for this image.