clusteralloc
int clusteralloc(struct msdosfsmount *, uint32_t, uint32_t, uint32_t *, uint32_t *);
error = clusteralloc(pmp, cn, count, &cn, &got);
error = clusteralloc(pmp, 0, 1, &newcluster, NULL);
int clusteralloc(struct msdosfsmount *, u_long, u_long,u_long *,u_long *);
error = clusteralloc(pmp, cn, count, &cn, &got);
error = clusteralloc(pmp, 0, 1, &newcluster, NULL);