f2fs: rename logical_to_blk and blk_to_logical

This patch renames two functions like below having u64.
 - logical_to_blk to bytes_to_blks
 - blk_to_logical to blks_to_bytes

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>