9237 "zpool add" fails for very large pools

illumos/illumos-gate@ae5ee1bd5e54409b4589217b160c8cf4b81e2fb8

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Approved by: Matt Ahrens <mahrens@delphix.com>
Author: Alan Somers <asomers@gmail.com>