zdb: include cloned blocks in block statistics

This gives zdb -b support for clone blocks.

Previously, it didn't know what clones were, so would count their space
allocation multiple times and then report leaked space (or, in debug,
would assert trying to claim blocks a second time).

This commit fixes those bugs, and reports the number of clones and the
space "used" (saved) by them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Closes #15123