xfs: add DAX file operations support

xfs: add DAX file operations support

Orabug: 22913653

Add the initial support for DAX file operations to XFS. This
includes the necessary block allocation and mmap page fault hooks
for DAX to function.

Note that there are changes to the splice interfaces to ensure that
for DAX splice avoids direct page cache manipulations and instead
takes the DAX IO paths for read/write operations.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
(cherry picked from commit 6b698edeeef00c127d73501b386590299f01327a)
Signed-off-by: Dan Duval <dan.duval@oracle.com>