radix-tree: add test for radix_tree_locate_item()

Add a unit test that provides coverage for the bug fixed in the commit
entitled "radix-tree: rewrite radix_tree_locate_item fix" from Hugh
Dickins. I've verified that this test fails before his patch due to
miscalculated 'index' values in __locate() in lib/radix-tree.c, and
passes with his fix.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ross Zwisler <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

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

···
+/* An empty file stub that allows radix-tree.c to compile. */

tools/testing/radix-tree/main.c

···
 	item_kill_tree(&tree);
 }
 
-void __locate_check(struct radix_tree_root *tree, unsigned long index,
+static void __locate_check(struct radix_tree_root *tree, unsigned long index,
 			unsigned order)
 {
 	struct item *item;
···
 	}
 }
 
+static void __order_0_locate_check(void)
+{
+	RADIX_TREE(tree, GFP_KERNEL);
+	int i;
+
+	for (i = 0; i < 50; i++)
+		__locate_check(&tree, rand() % INT_MAX, 0);
+
+	item_kill_tree(&tree);
+}
+
 static void locate_check(void)
 {
 	RADIX_TREE(tree, GFP_KERNEL);
 	unsigned order;
 	unsigned long offset, index;
+
+	__order_0_locate_check();
 
 	for (order = 0; order < 20; order++) {
 		for (offset = 0; offset < (1 << (order + 3));