Fix error merging r354116 from OpenZFS

When we merged 4c0883fb4af0d5565459099b98fcf90ecbfa1ca1 from OpenZFS (svn
r354116), there were some merge conflicts. One of those was resolved
incorrectly, causing "zfs receive" to fail to delete snapshots that a "zfs
send -R" stream has deleted.

This change corrects that merge conflict, and also reduces some harmless
diffs vis-a-vis OpenZFS that were also introduced by the same revision.
Direct commit to stable/12 because head has moved on to OpenZFS.

PR: 249438
Reported by: Dmitry Wagin <dmitry.wagin@ya.ru>
Reviewed by: mmacy
Sponsored by: Axcient