xarray: Add XArray load operation

The xa_load function brings with it a lot of infrastructure; xa_empty(),
xa_is_err(), and large chunks of the XArray advanced API that are used
to implement xa_load.

As the test-suite demonstrates, it is possible to use the XArray functions
on a radix tree.  The radix tree functions depend on the GFP flags being
stored in the root of the tree, so it's not possible to use the radix
tree functions on an XArray.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

tools/testing/radix-tree/test.h

@@ -34,6 +34,7 @@ int tag_tagged_items(struct radix_tree_root *, pthread_mutex_t *,
 			unsigned iftag, unsigned thentag);
 unsigned long find_item(struct radix_tree_root *, void *item);
 
+void xarray_tests(void);
 void tag_check(void);
 void multiorder_checks(void);
 void iteration_test(unsigned order, unsigned duration);