thp: enforcing constraints on file thps

[Zach O'Keefe <zokeefe@google.com>]

Hey All,

There are currently a number of paths where we can collapse file memory into
THPs:

1a) khugepaged - target of vma being processed
1b) khugepaged - other vma found mapping file, able to lock mmap_lock in
		 retract_page_tables()
1b) khugepaged - other vma found mapping file, deferred pte-mapped THP collapse,
		 processed in collapse_pte_mapped_thp()
2)  page fault finds hugepage in page cache + filemap_map_pages()

In terms of system-enforced THP constraints:

* vma flags + thps sysfs settings

  Checked in 1a. (1b now at least respects "never" THP mode after Yang Shi's
  cleanup series, but still doesn't respect "madvise" THP mode)

* MMF_DISABLE_THP

  Checked in 1a and 1b

I'm wondering if we should align these, and if so, in what direction? I would
argue that a process marked MMF_DISABLE_THP, or a vma marked VM_NOHUGEPAGE,
probably shouldn't be mapping at the pmd level, and that the appropriate checks
should be added in those paths.

Thanks,
Zach