Illumos #3743

3743 zfs needs a refcount audit
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:

https://www.illumos.org/issues/3743
illumos/illumos-gate@b287be1ba86043996f49b1cc34c80cc620f9b841

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775