rbd: feature bits should be re-read during header refresh
The kernel client was originally assuming feature bits would not change, and just reading an image's features once during mapping. The exclusive lock and object map features can now be added by userspace dynamically.
The features should be refreshed during the usual header refresh path. It's debatable what should happen when these are added to a mapped image... The image should at least go read only, and perhaps return EIO as well.
If the features are removed, and only compatible features are left, krbd could start accepting requests again, but it may not be worth the complexity of avoiding an unmap/remap.