f2fs: introduce macros to convert bytes and blocks in f2fs

This patch adds two macros for transition between byte and block offsets.
Currently, f2fs only supports 4KB blocks, so use the default size for now.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/file.c
Simple merge

diff --cc fs/f2fs/segment.c
Simple merge

diff --cc include/linux/f2fs_fs.h
Simple merge