MFV r301525:
7033 ustack helper should fault on bad return values
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: Bryan Cantrill <bryan@joyent.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Alex Wilson <alex.wilson@joyent.com>
illumos/illumos-gate@a2f72b65ebc430aaf277ad797e554bb4deba9b95
MFC after: 2 weeks