mm/zswap: use postorder iteration when destroying rbtree

static void zswap_frontswap_invalidate_area(unsigned type)

{

	struct zswap_tree *tree = zswap_trees[type];

	struct rb_node *node;

	struct zswap_entry *entry;

	struct zswap_entry *entry, *n;

	if (!tree)

		return;

	/* walk the tree and free everything */

	spin_lock(&tree->lock);

	/*

	 * TODO: Even though this code should not be executed because

	 * the try_to_unuse() in swapoff should have emptied the tree,

	 * it is very wasteful to rebalance the tree after every

	 * removal when we are freeing the whole tree.

	 *

	 * If post-order traversal code is ever added to the rbtree

	 * implementation, it should be used here.

	 */

	while ((node = rb_first(&tree->rbroot))) {

		entry = rb_entry(node, struct zswap_entry, rbnode);

		rb_erase(&entry->rbnode, &tree->rbroot);

	rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode) {

		zbud_free(tree->pool, entry->handle);

		zswap_entry_cache_free(entry);

		atomic_dec(&zswap_stored_pages);