ffs_isfreeblock
if (!ffs_isfreeblock(fs, blksfree, blkno)) {
int ffs_isfreeblock(struct fs *, unsigned char *, ufs_daddr_t);
if (!ffs_isfreeblock(fs, cg_blksfree_swap(cgp, needswap), fragno)) {
int ffs_isfreeblock(struct fs *, u_char *, int32_t);