Fix import wrong spare/l2 device when path change

Fix import wrong spare/l2 device when path change

If, for example, your aux device was /dev/sdc, but now the aux device is
removed and /dev/sdc points to other device. zpool import will still
use that device and corrupt it.

The problem is that the spa_validate_aux in spa_import, rather than
validate the on-disk label, it would actually write label to disk. We
remove them since spa_load_{spares,l2cache} seems to do everything we
need and they would actually validate on-disk label.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #6158