item_check_absent
item_check_absent(&tree, middle - down - 1);
item_check_absent(&tree, middle + up);
item_check_absent(&tree, 43);
item_check_absent(tree, index);
item_check_absent(tree, index);
item_check_absent(tree, index);
void item_check_absent(struct radix_tree_root *root, unsigned long index);