mini-os: mm: introduce generic page table walk function

mini-os: mm: introduce generic page table walk function

In x86 mm code there are multiple instances of page table walks for
different purposes.

Introduce a generic page table walker being able to cover the current
use cases. It will be used for other cases in future, too.

The page table walker needs some per-level data, so add a table for
that data. Merge it with the already existing pt_prot[] array.

Rewrite get_pgt() to use the new walker.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>