rbtree_get_free_block
block = rbtree_get_free_block(iter);
return rbtree_get_free_block(rb_last(root));
node = rbtree_get_free_block(parent);
block = rbtree_get_free_block(iter);
block = rbtree_get_free_block(node);
struct gpu_buddy_block *block = rbtree_get_free_block(rb);