MFV r348580: 9559 zfs diff handles files on delete queue in fromsnap poorly

illumos/illumos-gate@20633e304b57bc98f70fdb194081b7023adf527b

Reviewed by: Joshua M. Clulow <josh@sysmgr.org>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Paul Dagnelie <pcd@delphix.com>