These changes were added to help debug issue #9187.
Essentially, in the original bug, vdev_validate() seems to fails in
vdev_label_read_config() and prints "failed reading config". This could
happen because either:
1. The labels are actually corrupt and zio_wait() fails for all of them
2. The labels were discarded because they didn't pass the txg check.
Beyond 9187, having debug info when case 2 happens could be useful in
other scenarios, such as zpool import.
Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Approved by: Matt Ahrens <mahrens@delphix.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://illumos.org/issues/9189
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f6af1b7Closes#7533
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