rbd: extend the operation type
rbd: extend the operation type

It could only handle the read and write operations now,
extend it for the coming discard support.

Signed-off-by: Guangliang Zhao <a640ecc6a565666c11fd65fb44c17459dc60b214@gmail.com>
Reviewed-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>
Reviewed-by: Alex Elder <f429030cf5c0faf36fac3d102073b6e63a647baa@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs