zdb: add extra -T flag to show histograms of BRT refcounts
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16692
(cherry picked from commit 673efbbf5d160aea9a58f9eaac6d781d0975659a)