btrfs: don't use extent_root in iterate_extent_inodes

We are going to have many extent_roots soon, and we don't need a root
here necessarily as we're not modifying anything, we're just getting the
trans handle so we can have an accurate view of references, so use the
tree_root here.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -1969,7 +1969,7 @@ int iterate_extent_inodes(struct btrfs_fs_info *fs_info,
 			extent_item_objectid);
 
 	if (!search_commit_root) {
-		trans = btrfs_attach_transaction(fs_info->extent_root);
+		trans = btrfs_attach_transaction(fs_info->tree_root);
 		if (IS_ERR(trans)) {
 			if (PTR_ERR(trans) != -ENOENT &&
 			    PTR_ERR(trans) != -EROFS)