btrfs_calc_block_group_size_class
size_class = btrfs_calc_block_group_size_class(num_bytes);
size_class = btrfs_calc_block_group_size_class(min_size);
enum btrfs_block_group_size_class btrfs_calc_block_group_size_class(u64 size);
ffe_ctl->size_class = btrfs_calc_block_group_size_class(ffe_ctl->num_bytes);