ddt: move entry compression into ddt_zap

I think I can say with some confidence that anyone making a new storage
type in 2023 is doing their own thing with compression, not this.

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