Illumos #3104: eliminate empty bpobjs

3104 eliminate empty bpobjs
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Eric Schrock <eric.schrock@delphix.com>

References:

illumos/illumos-gate@f17457368189aa911f774c38c1f21875a568bdca
illumos changeset: 13782:8f78aae28a63
https://www.illumos.org/issues/3104

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