MFC r271781i (by will):
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