MFV r296306: 6604 harden DIF bounds checking

Reviewed by: Alex Wilson <alex.wilson@joyent.com>
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Bryan Cantrill <bryan@joyent.com>

illumos/illumos-gate@1c0cef67dba05c477dba779bc99224693e809a14

MFC after: 2 weeks