rbd: remove buggy rollback functionality

This doesn't interact with resizing well, since it doesn't set the
size of the device to the size at the snapshot. It's also an expensive
operation to be synchronous. Rollback can still be done with the
userspace rbd tool.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>

Documentation/ABI/testing/sysfs-bus-rbd

@@ -57,13 +57,6 @@ create_snap
 
 	 $ echo <snap-name> > /sys/bus/rbd/devices/<dev-id>/snap_create
 
-rollback_snap
-
-	Rolls back data to the specified snapshot. This goes over the entire
-	list of rados blocks and sends a rollback command to each.
-
-	 $ echo <snap-name> > /sys/bus/rbd/devices/<dev-id>/snap_rollback
-
 snap_*
 
 	A directory per each snapshot