Illumos #3145, #3212
    
    3145 single-copy arc
    3212 ztest: race condition between vdev_online() and spa_vdev_remove()
    
    Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
    Reviewed by: Adam Leventhal <ahl@delphix.com>
    Reviewed by: Eric Schrock <eric.schrock@delphix.com>
    Reviewed by: Justin T. Gibbs <gibbs@scsiguy.com>
    Approved by: Eric Schrock <eric.schrock@delphix.com>
    
    References:
      illumos-gate/commit/9253d63df408bb48584e0b1abfcc24ef2472382e
      illumos changeset: 13840:97fd5cdf328a
      https://www.illumos.org/issues/3145
      https://www.illumos.org/issues/3212
    
    Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
    Closes #989
    Closes #1137