ffs_clrblock
ffs_clrblock(fs, blksfree, (long)bno);
void ffs_clrblock(struct fs *, u_char *, ufs1_daddr_t);
ffs_clrblock(fs, blksfree, (long)blkno);
void ffs_clrblock(struct fs *, uint8_t *, ufs1_daddr_t);
ffs_clrblock(fs, blksfree, (long)blkno);
ffs_clrblock(fs, blksfree_swap, (long)blkno);
void ffs_clrblock(struct fs *, u_char *, int32_t);