rgw: set null version object acl issues

rgw: set null version object acl issues

1.set null version object acl will create empty index
RGWRados::set_attrs did not clear instance, so index prepare, complete got instance=null,
which lead to empty index 1000_<obj>_i_null.
there is no harm to create empty index, but listomapkeys to find that key.

2.if object is exist with versioned key, we can set none exists null version object
order:
1) enable bucket version
2) put obj
3) disable bucket version
4) set versoned_id=null acl will succeed which should not

Fixes: http://tracker.ceph.com/issues/36763
Signed-off-by: Tianshan Qu <tianshan@xsky.com>