Illumos 3139 - zdb dies when it tries to determine path of unlinked file

3139 zdb dies when it tries to determine path of unlinked file
Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:

https://github.com/illumos/illumos-gate/commit/1ce39b5
https://www.illumos.org/issues/3139

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>