500 rbd: add reference counting to rbd_spec With layered images we'll share rbd_spec structures, so add a reference count to it. It neatens up some code also. A silly get/put pair is added to the alloc routine just to avoid "defined but not used" warnings. It will go away soon. Signed-off-by: Alex Elder Reviewed-by: Josh Durgin ÓzÂΖ+x