Illumos 5814 - bpobj_iterate_impl(): Close a refcount leak iterating on a sublist.

5814 bpobj_iterate_impl(): Close a refcount leak iterating on a sublist.
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:

https://www.illumos.org/issues/5814
https://github.com/illumos/illumos-gate/commit/b67dde11

Ported-by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3368