btrfs: use btrfs_for_each_slot in find_first_block_group

	struct btrfs_root *root = btrfs_block_group_root(fs_info);

	int ret;

	struct btrfs_key found_key;

	struct extent_buffer *leaf;

	int slot;

	ret = btrfs_search_slot(NULL, root, key, path, 0, 0);

	if (ret < 0)

		return ret;

	while (1) {

		slot = path->slots[0];

		leaf = path->nodes[0];

		if (slot >= btrfs_header_nritems(leaf)) {

			ret = btrfs_next_leaf(root, path);

			if (ret == 0)

				continue;

			if (ret < 0)

				goto out;

			break;

		}

		btrfs_item_key_to_cpu(leaf, &found_key, slot);

	btrfs_for_each_slot(root, key, &found_key, path, ret) {

		if (found_key.objectid >= key->objectid &&

		    found_key.type == BTRFS_BLOCK_GROUP_ITEM_KEY) {

			ret = read_bg_from_eb(fs_info, &found_key, path);

			break;

			return read_bg_from_eb(fs_info, &found_key, path);

		}

		path->slots[0]++;

	}

out:

	return ret;

}