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/linux/bug.h

@@ -1 +1,2 @@
+#include <stdio.h>
 #include "asm/bug.h"

tools/testing/radix-tree/linux/kconfig.h

@@ -0,0 +1 @@
+#include "../../../../include/linux/kconfig.h"