Fix failing > 4MB range requests through radosgw S3 API.

When a range request is made for more than rgw_get_obj_max_req_size
bytes the first returned chunk sets 'ret' to STATUS_PARTIAL_CONTENT and
all remaining chunks behave as if there is an error state and only
return a minimal header.

Fix this by passing STATUS_PARTIAL_CONTENT to set_req_state_err, but
leave the 'ret' member variable untouched.

Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit c83a01d4e8dcd26eec24c020c5b79fcfa4ae44a3)