Revert "MFC kern: cpuset: properly rebase when attaching to a jail"

This behavior change is too invasive to be made between minor versions,
back it out in stable/12 -- it will be first introduced in 13.0.

The cpuset test has been adjusted to account for the legacy behavior,
with a note added as to why it's different and doesn't work if run
as-is on 13.0.

This reverts commit 7bb960ce6447bd535e0fbb648e4d9edbb1dc067f.