fix nspace assignment in LFNIndex::lfn_parse_object_name

The unused ns variable is assigned to the nspace data member of the
parsed hobject_t. It would be nicer to use a constructor accepting
the namespace in argument.

http://tracker.ceph.com/issues/4597 refs #4597

Signed-off-by: Loic Dachary <loic@dachary.org>