OpenZFS 7496 - cmp_ds_cont has never worked

OpenZFS 7496 - cmp_ds_cont has never worked

Authored by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7496
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/5dc1fd7
Closes #5781