Illumos 5150 - zfs clone of a defer_destroy snapshot causes strangeness

When a clone is created of a snapshot that has been marked for
deferred destroy (with "zfs destroy -d"), the clone "inherits" the
defer_destroy flag from the origin, and any snapshots of the clone
"inherit" the defer_destroy flag from the clone. This causes a strange
situation where the clone's snapshots are marked for defer_destroy but
they have no holds or clones. If the clone's snapshot gets a hold or
clone, which is then deleted, we will honor the incorrectly-set
defer_destroy flag and delete the snapshot!

Steps to reproduce:

  * zpool create test c1t1d0
  * zfs create test/fs
  * zfs snapshot test/fs@a
  * zfs clone test/fs@a test/clone
  * zfs destroy -d test/fs@a
  * zfs clone test/fs@a test/clone2
  * zfs snapshot test/clone2@a
  * zfs hold hld test/clone2@a
  * zfs release hld test/clone2@a
  * zfs list -r -t all test

  <test/clone2@a has been destroyed>

We noticed that this causes dcenter to get very confused, because it
treats snapshots that are marked defer_destroy as not existing. So it
won't see any snapshots of the clone that's marked defer_destroy.

Reviewed by: George Wilson george.wilson@delphix.com
Reviewed by: Christopher Siden christopher.siden@delphix.com
Reviewed by: Max Grossman max.grossman@delphix.com

Original author: Matthew Ahrens

References:
  https://www.illumos.org/projects/illumos-gate//issues/5150
  https://reviews.csiden.org/r/93/

Ported by: Turbo Fredriksson <turbo@bayour.com>