jbd2: clean up checksum verification in do_one_pass()

Remove the unnecessary chksum_err and checksum_seen variables as well as
some redundant code to make the function easier to understand.

[ With changes suggested by jack@ and tytso@ ]

Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200819122955.33526-1-luoshijie1@huawei.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>