lib/checksum: fix one more thinko

When do_csum gets unaligned data, we really need to treat
the first byte as an even byte, not an odd byte, because
we swap the two halves later.

Found by Mike's checksum-selftest module.

Reported-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>