thp: support file pages in zap_huge_pmd()

split_huge_pmd() for file mappings (and DAX too) is implemented by just
clearing pmd entry as we can re-fill this area from page cache on pte
level later.

This means we don't need deposit page tables when file THP is mapped.
Therefore we shouldn't try to withdraw a page table on zap_huge_pmd()
file THP PMD.

Link: http://lkml.kernel.org/r/1466021202-61880-14-git-send-email-kirill.shutemov@linux.intel.com


Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>