ffs_isfreeblock
if (!ffs_isfreeblock(fs, blksfree, fragno)) {
int ffs_isfreeblock(struct fs *, u_char *, int32_t);
if (!ffs_isfreeblock(fs, cg_blksfree(cgp, needswap), fragno)) {
int ffs_isfreeblock(struct fs *, u_char *, int32_t);