thp: fix zap_huge_pmd() for DAX

thp: fix zap_huge_pmd() for DAX

Orabug: 22913653

The original DAX code assumed that pgtable_t was a pointer, which isn't
true on all architectures.  Restructure the code to not rely on that
assumption.

[willy@linux.intel.com: further fixes integrated into this patch]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit da146769004e1dd5ed06853e6d009be8ca675d5f)
Signed-off-by: Dan Duval <dan.duval@oracle.com>