rbd: add an rbd features field

Record the features values for each rbd image and each of its
snapshots.  This is really something that only becomes meaningful
for version 2 images, so this is just putting in place code
that will form common infrastructure.

It may be useful to expand the sysfs entries--and therefore the
information we maintain--for the image and for each snapshot.
But I'm going to hold off doing that until we start making
active use of the feature bits.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

Documentation/ABI/testing/sysfs-bus-rbd

@@ -25,6 +25,10 @@ client_id
 
 	The ceph unique client id that was assigned for this specific session.
 
+features
+
+	A hexadecimal encoding of the feature bits for this image.
+
 major
 
 	The block device major number.
@@ -78,4 +82,7 @@ snap_size
 
 	The size of the image when this snapshot was taken.
 
+snap_features
+
+	A hexadecimal encoding of the feature bits for this snapshot.