OpenZFS 6412 - zfs receive: -u can be ignored sometimes

Authored by: Andriy Gapon <andriy.gapon@clusterhq.com>
Reviewed by: - Matthew Ahrens <mahrens@delphix.com>
Reviewed by: - Paul Dagnelie <pcd@delphix.com>
Approved by: - Richard Lowe <richlowe@richlowe.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/6412
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/9185393
Closes #5659