zdb: Fix false leak report for BRT objects

Fix a misreport in 'zdb -d' where it falsely marked
BRT objects as leaked.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Yuxin Wang <yuxinwang9999@gmail.com>
Closes #15882