rgw, cls_rgw: don't remove olh objects

A single head object can contain both olh data, and object data. Don't
remove object if it has olh information on it, just reset the other
info.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
cleanup, squash
cls_rgw: allow olh linking to null instance objects

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: set olh if object has been versioned

Bucket could have versioning suspended, but object has been versioned,
so once object has olh pointer in it, we need to continue maintainig it.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>