Illumos 6451 - ztest fails due to checksum errors

6451 ztest fails due to checksum errors
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Jorgen Lundman <lundman@lundman.net>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/issues/6451
https://github.com/illumos/illumos-gate/commit/f9eb9fd

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>