optimization, this will only affect UFS2 filesystems.

> /*
>  * If DO_INODES is defined, we look at the inode list in each cylinder group
>  * and try to make further space reducing optimizations.  If there are
>  * uninitialized inodes (UFS2 only) we add those blocks to the skip list.
>  *
>  * If CLEAR_FREE_INODES is also defined, we make a more dubious optimization.
>  * Initialized but free inodes will go into the image data, but we first zero
>  * everything except the (usually randomized) generation number in an attempt
>  * to reduce the compressed size of the data.
>  */
> #define DO_INODES
> #undef CLEAR_FREE_INODES