mm/gup: fix gup_pmd_range() for dax

mm/gup: fix gup_pmd_range() for dax

For dax pmd, pmd_trans_huge() returns false but pmd_huge() returns true
on x86.  So the function works as long as hugetlb is configured.
However, dax doesn't depend on hugetlb.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yu Zhao <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: "Michael S . Tsirkin" <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>