OpenZFS 6101 - attempt to lzc_create() a filesystem under a volume results in a panic

OpenZFS 6101 - attempt to lzc_create() a filesystem under a volume results in a panic

Authored by: Andriy Gapon <avg@FreeBSD.org>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
When querying ZPL properties verify that the objset is of type
DMU_OST_ZFS.

OpenZFS-issue: https://www.illumos.org/issues/6101
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/ce2243a
Closes #6015