Simplify dir calcs

[thejpster]

We do this a lot. Can it be simplfied or made into a function:

                let mut first_dir_block_num = match dir.cluster {
                    ClusterId::ROOT_DIR => self.lba_start + fat16_info.first_root_dir_block,
                    _ => self.cluster_to_block(dir.cluster),
                };
                let dir_size = match dir.cluster {
                    ClusterId::ROOT_DIR => {
                        let len_bytes =
                            u32::from(fat16_info.root_entries_count) * OnDiskDirEntry::LEN_U32;
                        BlockCount::from_bytes(len_bytes)
                    }
                    _ => BlockCount(u32::from(self.blocks_per_cluster)),
                };