dax: add huge page fault support

dax: add huge page fault support

Orabug: 22913653

This is the support code for DAX-enabled filesystems to allow them to
provide huge pages in response to faults.

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 844f35db1088dd1a9de37b53d4d823626232bd19)
Signed-off-by: Dan Duval <dan.duval@oracle.com>