f2fs: compress: fix compression chksum

commit 75e91c88 upstream.

This patch addresses minor issues in compression chksum.

Fixes: b28f047b

 ("f2fs: compress: support chksum")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>