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

If bpobj_space() returned non-zero here, the sublist would have been
left open, along with the bonus buffer hold it requires.  This call
does not invoke any calls to bpobj_close() itself.

This bug doesn't have any known vector, but was found on inspection.

MFC after:	1 week
Sponsored by:	Spectra Logic
Affects:	All ZFS versions starting 21 May 2010 (illumos cde58dbc)
MFSpectraBSD:	r1050998 on 2014/03/26