MFC r336943:
MFV r336942: 9189 Add debug to vdev_label_read_config when txg check fails

illumos/illumos-gate@b6bf6e1540f30bd97b8d6e2c21d95e17841e0f23

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Matt Ahrens <mahrens@delphix.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>