MFV r254079:

Illumos ZFS issues:
  3957 ztest should update the cachefile before killing itself
  3958 multiple scans can lead to partial resilvering
  3959 ddt entries are not always resilvered
  3960 dsl_scan can skip over dedup-ed blocks if
       physical birth != logical birth
  3961 freed gang blocks are not resilvered and can cause pool to suspend
  3962 ztest should print out zfs debug buffer before exiting