6841 Undirty freed spill blocks

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Tim Chase <tim@chase2k.com>

openzfs/openzfs@445e67805dd2ca6c3a2363b2ec9e163c62370233