MFV r288408:
6266 harden dtrace_difo_chunksize() with respect to malicious DIF
illumos/illumos-gate@395c7a3dcfc66b8b671dc4b3c4a2f0ca26449922
Reviewed by: Alex Wilson <alex.wilson@joyent.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: Bryan Cantrill <bryan@joyent.com>
MFC after: 1 week