zdb: Fix big parameter passed by value

This is not in performance critical code, but static analyzers will
complain about it, so lets switch to pass by pointer here.

Reported-by: Coverity (CID-1524384)
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14263