MFV r294798:
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>

illumos/illumos-gate@a443cc80c742af740aa82130db840f02b4389365