OpenZFS 7762 - avoid division by zero in property_alias_001_pos

OpenZFS 7762 - avoid division by zero in property_alias_001_pos

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/7762
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/ebaf15cb
Closes #5798