mm: export various functions for the benefit of DAX

mm: export various functions for the benefit of DAX

Orabug: 22913653

To use the huge zero page in DAX, we need these functions exported.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit fc43704437ebe40f642ac53f7ee73661fe74e6b8)
Signed-off-by: Dan Duval <dan.duval@oracle.com>