MFV r294802: 6334 Cannot unlink files when over quota

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Simon Klinkert <simon.klinkert@gmail.com>

illumos/illumos-gate@6575bca01367958c7237253d88e5fa9ef0b1650a