Remove the residual geometry code and datastructures from FFS (keep some kind of allocation groups but without most of what cylinder groups now have) and replace blocks and fragments with extents, yielding a much simpler filesystem well suited for modern disks.

Note that this results in a different on-disk format and will need to be a different file system type.

The result would be potentially useful to other operating systems beyond just NetBSD, since UFS/FFS is used in so many different kernels.