rbd: define inbound data size for method ops

When rbd creates an object request containing an object method call
operation it is passing 0 for the size.  I originally thought this
was because the length was not needed for method calls, but I think
it really should be supplied, to describe how much space is
available to receive response data.  So provide the supplied length.

This resolves:
    http://tracker.ceph.com/issues/4659



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