mm: thp: kill __transhuge_page_enabled()

The page fault path checks THP eligibility with __transhuge_page_enabled()
which does the similar thing as hugepage_vma_check(), so use
hugepage_vma_check() instead.

However page fault allows DAX and !anon_vma cases, so added a new flag,
in_pf, to hugepage_vma_check() to make page fault work correctly.

The in_pf flag is also used to skip shmem and file THP for page fault
since shmem handles THP in its own shmem_fault() and file THP allocation
on fault is not supported yet.

Also remove hugepage_vma_enabled() since hugepage_vma_check() is the only
caller now, it is not necessary to have a helper function.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Reviewed-by: Zach O'Keefe <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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