illumos gate

Not sure if this one due to a restriction on illumos not present in FreeBSD but we've found on a number of occasions that zpool.cache is out of date for root pools.

The reason for this is that spa_config_update doesn't call spa_config_sync if the pool is the root pool.

This causes various issues and inconsistencies when running tools which depend on the contents of zpool.cache such as zdb reporting out of date information about the pool.

I've just committed the attached patch to FreeBSD which fixes this issue, with no perceived side effects.