f2fs: clean up w/ F2FS_{BLK_TO_BYTES,BTYES_TO_BLK}

f2fs doesn't support different blksize in one instance, so
bytes_to_blks() and blks_to_bytes() are equal to F2FS_BYTES_TO_BLK
and F2FS_BLK_TO_BYTES, let's use F2FS_BYTES_TO_BLK/F2FS_BLK_TO_BYTES
instead for cleanup.

Reviewed-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>