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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NBrian Foster <bfoster@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/xfs/xfs_dir2_readdir.c

@@ -669,7 +669,7 @@ xfs_readdir(
 	struct dir_context	*ctx,
 	size_t			bufsize)
 {
-	struct xfs_da_args	args = {0};
+	struct xfs_da_args	args = { NULL };
 	int			rval;
 	int			v;
 	uint			lock_mode;