SET_TX_OPAQUE
txbd->tx_bd_opaque = SET_TX_OPAQUE(bn, txr, prod, 2 + last_frag);
txbd->tx_bd_opaque = SET_TX_OPAQUE(bp, txr, prod, 2);
txbd->tx_bd_opaque = SET_TX_OPAQUE(bp, txr, prod, 2 + last_frag);
txbd->tx_bd_opaque = SET_TX_OPAQUE(bp, txr, prod, 1 + num_frags);