xfs: add DAX truncate support

xfs: add DAX truncate support

Orabug: 22913653

When we truncate a DAX file, we need to call through the DAX page
truncation path rather than through block_truncate_page() so that
mappings and block zeroing are all handled correctly. Otherwise,
truncate does not need to change.

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 9969441f9f86a8a7de8c36514fa789e5f5d83145)
Signed-off-by: Dan Duval <dan.duval@oracle.com>