ddt: remove struct names and forward declarations

Things get confused when there's more than one name for a thing.

Note that we don't do this for ddt_object_t, ddt_histogram_t and
ddt_stat_t because they're part of the public ZFS interface.

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