ddt: remove DDE_GET_NDVAS macro

It was a weird and confusing name, because it wasn't actually returning
the number of DVAs in the entry (as in, in the value/phys part) but the
maximum number of possible DVAs in a BP generated from the entry, based
on the encrypt bit in the key. This is unlike the similarly named
BP_GET_NDVAS, which really does return the number of DVAs.

Since its only used in this one place, and for a specific purpose, it
seemed more sensible to just write it in-place and remove the name.

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