rbd: whitelist RBD_FEATURE_OPERATIONS feature bit

This feature bit restricts older clients from performing certain
maintenance operations against an image (e.g. clone, snap create).
krbd does not perform maintenance operations.

Cc: stable@vger.kernel.org
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>