xarray: Add definition of struct xarray

This is a direct replacement for struct radix_tree_root.  Some of the
struct members have changed name; convert those, and use a #define so
that radix_tree users continue to work without change.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>

tools/testing/radix-tree/xarray.c

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * xarray.c: Userspace shim for XArray test-suite
+ * Copyright (c) 2018 Matthew Wilcox <willy@infradead.org>
+ */
+
+#include "../../../lib/xarray.c"