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

References:
  https://www.illumos.org/issues/5178
  https://reviews.csiden.org/r/105/

Ported by: Turbo Fredriksson <turbo@bayour.com>