dax: don't use set_huge_zero_page()

dax: don't use set_huge_zero_page()

Orabug: 22913653

This is another place where DAX assumed that pgtable_t was a pointer.
Open code the important parts of set_huge_zero_page() in DAX and make
set_huge_zero_page() static again.

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 d295e3415a88ae63a37a22652808b20c7fcb970e)
Signed-off-by: Dan Duval <dan.duval@oracle.com>