9738 9112 broke third block copy allocations within one metaslab group
illumos/illumos-gate@b86e7e3f0e50748bb5bb5cc91632d72ff17f08dd
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Alexander Motin <mav@FreeBSD.org>