Introduce the ability for khugepaged to collapse to different mTHP sizes.
While scanning a PMD range for potential hugepage collapse, track pages
in MIN_MTHP_ORDER chunks. Each bit represents a fully utilized region of
order MIN_MTHP_ORDER ptes.
With this bitmap we can determine which mTHP sizes would be the most
efficient to collapse to if the PMD collapse is not suitible.
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>