OpenZFS 9559 - zfs diff handles files on delete queue in fromsnap poorly

Authored by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Joshua M. Clulow <josh@sysmgr.org>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/9559
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d7e45412
Closes #8211