zdb: fix BRT dump (#16335)

BRT refcounts are stored as eight uint8_ts rather than a single
uint64_t. This means that za_first_integer is only the first byte, so
max 256. This fixes it by doing a lookup for the whole value.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>