Illumos 5178 - zdb -vvvvv on old-format pool fails in dump_deadlist()

5178 zdb -vvvvv on old-format pool fails in dump_deadlist()
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Richard Elling <richard.elling@gmail.com>
Reviewed by: Alek Pinchuk <alek.pinchuk@nexenta.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  https://www.illumos.org/issues/5178
  https://github.com/illumos/illumos-gate/commit/90c76c6

Ported by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2713