MFV r319743: 8108 zdb -l fails to read labels 2 and 3

illumos/illumos-gate@22c8b9583d07895c16549075a53668d7bc988cf3
https://github.com/illumos/illumos-gate/commit/22c8b9583d07895c16549075a53668d7bc988cf3

https://www.illumos.org/issues/8108

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>

MFC after: 3 weeks