The conversion is quite straightforward, just replace the page API to
the corresponding folio API.  migrate_pages() and unmap_and_move()
mostly work with folios (head pages) only.

Changelog:

v1 -> v2:

- Rebased on mm-unstable.

- Added reviewed-by.

rfc -> v1:

- Change API to test transhuge and hugetlb, per Matthew and Zi's comments.

- Make THP related statistics for THP only, per Matthew's comments.

Best Regards,
Huang, Ying