f2fs: clarify compress level bit offset

commit 3fde13f8 ("f2fs: compress: support compress level") introduce
compress level, which macro(COMPRESS_LEVEL_OFFSET) is 8, But use wrong
comment about compress level.

Let's fix it.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>