FreeBSD rS271776 - Persist vdev_resilver_txg changes

Persist vdev_resilver_txg changes to avoid panic caused by validation
vs a vdev_resilver_txg value from a previous resilver.

Authored-by: smh <smh@FreeBSD.org>
Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/5154
FreeBSD-issue: https://reviews.freebsd.org/rS271776
FreeBSD-commit: https://github.com/freebsd/freebsd/commit/c3c60bf
Closes #4790