rgw: only look at prefetched data if we actually prefetched

Fixes: #7903
Since we didn't prefetch data then we couldn't rely on the data to
actually exist there. In that case just move on and read the object.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: move max_chunk_size initialization

RGWRados::initialize() is not called when doing
RGWRados::get_raw_storage_provider(). This was the culprit for issue

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1560 from ceph/wip-7903

Wip 7903

Reviewed-by: Sage Weil <sage@inktank.com>