ioat(4): Simplify by removing dynamic scaling

This paves the way for a contiguous descriptor array.

A contiguous descriptor array has the benefit that we can make strong
assertions about whether an address is a valid descriptor or not. The
other benefit is that future generations of I/OAT hardware will require
a contiguous descriptor array anyway. The downside is that after system
boot, big chunks of contiguous memory is much harder to find. So
dynamic scaling after boot is basically impossible.

Reviewed by: markj
Sponsored by: Dell EMC Isilon