ffs_clrblock
ffs_clrblock(fs, blksfree, blkno);
void ffs_clrblock(struct fs *, u_char *, daddr_t);
void ffs_clrblock(struct fs *, u_char *, daddr_t);
ffs_clrblock(fs, blksfree, (long)blkno);
void ffs_clrblock(struct fs *, u_char *, int32_t);