netmap_obj_offset
netmap_obj_offset(&(n)->pools[NETMAP_IF_POOL], (v))
netmap_obj_offset(&(n)->pools[NETMAP_RING_POOL], (v)))
(netmap_obj_offset(&(n)->pools[NETMAP_BUF_POOL], (v)) / NETMAP_BDG_BUF_SIZE(n))