ddt: simplify entry load and flags

Only a single bit is needed to track entry state, and definitely not two
whole bytes. Some light refactoring in ddt_lookup() is needed to support
this, but it reads a lot better 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