vtnet_rx_alloc_buf
vtnet_rx_alloc_buf(struct vtnet_softc *, int , struct mbuf **);
m_new = vtnet_rx_alloc_buf(sc, nreplace, &m_tail);
m_new = vtnet_rx_alloc_buf(sc, 1, NULL);
m = vtnet_rx_alloc_buf(sc, sc->vtnet_rx_nmbufs, NULL);