Old csum type check is wrong and can't catch csum_type 1(not supported).

Fix it to avoid hostile 0 division.
Reported-by: NLukas Lueg <lukas.lueg@gmail.com>
Signed-off-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>