[$] Multi-size THP creation, two different ways

Huge pages can increase the performance of many programs, but they can also
have unfortunate performance impacts of their own. Over the last few
years, multi-size transparent huge pages (mTHPs) have increasingly been
seen as a happy medium that bring the benefits of huge pages at a lower cost.
The system cannot benefit from mTHPs, though, if it does not create them;
two developers have independently posted patches to enable the creation of
mTHPs in the background.