netmap_get_na
error = netmap_get_na(nmr, &na, 1 /* create */);
error = netmap_get_na(nmr, &na, 1 /* create */); /* keep reference */
error = netmap_get_na(nmr, &na, 0 /* don't create */); /* keep reference */
int netmap_get_na(struct nmreq *nmr, struct netmap_adapter **na, int create);
error = netmap_get_na(nmr, &na, 1 /* create if not exists */);
error = netmap_get_na(nmr, &na, 0 /* don't create */);