Btrfs: only add roots if necessary in find_parent_nodes()

find_all_leafs() dosen't need add all roots actually, add roots only
if we need, this can avoid unnecessary ulist dance.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/backref.c

@@ -965,7 +965,7 @@ again:
 	while (!list_empty(&prefs)) {
 		ref = list_first_entry(&prefs, struct __prelim_ref, list);
 		WARN_ON(ref->count < 0);
-		if (ref->count && ref->root_id && ref->parent == 0) {
+		if (roots && ref->count && ref->root_id && ref->parent == 0) {
 			/* no parent == root of tree */
 			ret = ulist_add(roots, ref->root_id, 0, GFP_NOFS);
 			if (ret < 0)
@@ -1061,22 +1061,14 @@ static int btrfs_find_all_leafs(struct btrfs_trans_handle *trans,
 				u64 time_seq, struct ulist **leafs,
 				const u64 *extent_item_pos)
 {
-	struct ulist *tmp;
 	int ret;
 
-	tmp = ulist_alloc(GFP_NOFS);
-	if (!tmp)
-		return -ENOMEM;
 	*leafs = ulist_alloc(GFP_NOFS);
-	if (!*leafs) {
-		ulist_free(tmp);
+	if (!*leafs)
 		return -ENOMEM;
-	}
 
 	ret = find_parent_nodes(trans, fs_info, bytenr,
-				time_seq, *leafs, tmp, extent_item_pos);
-	ulist_free(tmp);
-
+				time_seq, *leafs, NULL, extent_item_pos);
 	if (ret < 0 && ret != -ENOENT) {
 		free_leaf_list(*leafs);
 		return ret;