xfs: add DAX block zeroing support

xfs: add DAX block zeroing support

Orabug: 22913653

Add initial support for DAX block zeroing operations to XFS. DAX
cannot use buffered IO through the page cache for zeroing, nor do we
need to issue IO for uncached block zeroing. In both cases, we can
simply call out to the dax block zeroing function.

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 4f69f578a87d39c20b1ff70005a125e4594c3de8)
Signed-off-by: Dan Duval <dan.duval@oracle.com>