Illumos 6292 - exporting a pool while an async destroy

Illumos 6292 - exporting a pool while an async destroy

6292 exporting a pool while an async destroy is running can leave
entries in the deferred tree
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Fabian Keil <fk@fabiankeil.de>
Approved by: Gordon Ross <gordon.ross@nexenta.com>

References:
  https://www.illumos.org/issues/6292
  https://github.com/illumos/illumos-gate/commit/a443cc8

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