Illumos #3090 and #3102

3090 vdev_reopen() during reguid causes vdev to be treated as corrupt
3102 vdev_uberblock_load() and vdev_validate() may read the wrong label

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:

illumos/illumos-gate@dfbb943217bf8ab22a1a9d2e9dca01d4da95ee0b
illumos changeset: 13777:b1e53580146d
https://www.illumos.org/issues/3090
https://www.illumos.org/issues/3102

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #939