Illumos 5393 - spurious failures from dsl_dataset_hold_obj()

5393 spurious failures from dsl_dataset_hold_obj()
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Steven Hartland <killing@multiplay.co.uk>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/5393
https://github.com/illumos/illumos-gate/commit/e1f3c20

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3403