zdb: show bp in uberblock dump

Just another useful nugget of info in times of strife.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16667