xfs: fix xfs_da_args sparse warning in xfs_readdir

The kbuild test robot reported:

>> fs/xfs/xfs_dir2_readdir.c:672:41: sparse: Using plain integer as NULL pointer

Fix it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>