Teach zdb about DMU_OT_ERROR_LOG objects

With the persistent error log feature we need to account for
spa_errlog_{scrub, last} containing mappings to other error log objects,
which need to be marked as in-use as well.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14442
Closes #14434