[PATCH] ZERO_PAGE again v5.

Updated from v4 as
  - avoid to add new arguments to vm_normal_page().
    vm_normal_page() always returns NULL if ZERO_PAGE is found.
  - follow_page() directly handles pte_special and ANON_ZERO_PAGE.

Then, amount of changes are reduced. Thanks for advices.

Concerns pointed out:
  - Does use_zero_page() cover all cases ?
    I think yes..
  - All get_user_pages() callers, which may find ZERO_PAGE is safe ?
    need tests.
  - All follow_pages() callers, which may find ZERO_PAGE is safe ?
    I think yes.

Thanks,
-Kame

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