ddt: only create tables for dedup-capable checksums

Most values in zio_checksum can never be used for dedup, partly because
the dedup= property only offers a limited list, but also some values (eg
ZIO_CHECKSUM_OFF) aren't real and will never be seen.

A true flag would be better than a hardcoded list, but thats more
cleanup elsewhere than I want to do right now.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15887