mbox_channel_idx
sc->sc_mc = mbox_channel_idx(faa->fa_node, 0, NULL);
sc->sc_mc = mbox_channel_idx(node, 0, NULL);
sc->sc_mc = mbox_channel_idx(sc->sc_node, 0, NULL);
return mbox_channel_idx(node, idx, client);
struct mbox_channel *mbox_channel_idx(int, int, struct mbox_client *);