Illumos 5311 - traverse_dnode may report success when it should not

5311 traverse_dnode may report success when it should not
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Will Andrews <willa@spectralogic.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  illumos/illumos-gate@2a89c2c59b7c2beb2373c14368cbe7e32af6ffc1
  https://www.illumos.org/issues/5311

Ported by: DHE <git@dehacked.net>