MFV r348573: 9993 zil writes can get delayed in zio pipeline

illumos/illumos-gate@2258ad0b755b24a55c6173b1e6bb6188389f72dd

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: George Wilson <george.wilson@delphix.com>