ddt: slim down ddt_entry_t

This slims down the in-memory entry to as small as it can be. The
IO-related parts are made into a separate entry, since they're
relatively rarely needed.

The variable allocation for dde_phys is to support the upcoming flat
format.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
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 #15893