rbd: ignore zero-length requests

The old request code simply ignored zero-length requests.  We should
still operate that same way to avoid any changes in behavior.  We
can implement handling for special zero-length requests separately
(see http://tracker.ceph.com/issues/4236).

Add some assertions based on this new constraint.

This resolves:
    http://tracker.ceph.com/issues/4237



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