xfs: add DAX IO path support

xfs: add DAX IO path support

Orabug: 22913653

DAX does not do buffered IO (can't buffer direct access!) and hence
all read/write IO is vectored through the direct IO path.  Hence we
need to add the DAX IO path callouts to the direct IO
infrastructure.

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 6e1ba0bcb84b3f97616feb07c27f974509ba57be)
Signed-off-by: Dan Duval <dan.duval@oracle.com>