btrfs_lookup_first_block_group
return btrfs_lookup_first_block_group(fs_info, next_bytenr);
block_group = btrfs_lookup_first_block_group(info, 0);
struct btrfs_block_group *btrfs_lookup_first_block_group(
block_group = btrfs_lookup_first_block_group(fs_info, bytenr);
cache = btrfs_lookup_first_block_group(fs_info, range->start);
bg = btrfs_lookup_first_block_group(fs_info, 0);