From: Brian Behlendorf <behlendorf1@llnl.gov>
Subject: [PATCH] fix bpobj_close

During spa_load the spl->spa_deferred_bpobj maybe be opened and closed
multiple times.  It's critical that when the object is closed the
bpo->bpo_object is set to zero to indicate the object is closed.
If it's not during spl_load_retry the spl->spa_deferred_bpobj can
be closes twice resulting in a NULL deref.

This appears to have been fixed upstream the same way.