Illumos #3122 zfs destroy filesystem should prefetch blocks

3122 zfs destroy filesystem should prefetch blocks
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  illumos/illumos-gate@b4709335aa83dcbfd0dba33c9be21fcabebd28e4
  https://www.illumos.org/issues/3122

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1565
Illumos #3498 panic in arc_read()

3498 panic in arc_read(): !refcount_is_zero(&pbuf->b_hdr->b_refcnt)
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  illumos/illumos-gate@1b912ec7100c10e7243bf0879af0fe580e08c73d
  https://www.illumos.org/issues/3498

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1249