9616 Bogus error when attempting to set property on read-only pool

illumos/illumos-gate@f62db44dbcda5dd786bb821f1e6fd3ca2e6d4391

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andrew Stormont <astormont@racktopsystems.com>