ddt: lift dedup stats out to separate file

We want to add other kinds of dedup-related objects and keep stats for
them. This makes those functions easier to use from outside ddt.c.

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