7263 deeply nested nvlist can overflow stack

illumos/illumos-gate@9ca527c3d3dfa7c8f304b34a9e03b5eddace838f

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>