9962 zil_commit should omit cache thrash

illumos/illumos-gate@cab3a55e158118937e07d059c46f1bc14d1f254d

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Approved by: Joshua M. Clulow <josh@sysmgr.org>
Author: Prakash Surya <prakash.surya@delphix.com>