s3: drain request body before error response (#11334)

* s3: drain request body before error response

* s3: keep oversized request bodies drainable

weed/s3api/s3api_server.go

@@ -709,10 +709,8 @@ func (s3a *S3ApiServer) UnifiedPostHandler(w http.ResponseWriter, r *http.Reques
 	// Save the body first so we can restore it for STS handler signature verification
 	var bodyBytes []byte
 	if r.Body != nil {
-		// Limit body size to prevent DoS attacks
-		r.Body = http.MaxBytesReader(w, r.Body, iamRequestBodyLimit)
 		var err error
-		bodyBytes, err = io.ReadAll(r.Body)
+		bodyBytes, err = readRequestBody(r, iamRequestBodyLimit)
 		if err != nil {
 			glog.Errorf("failed to read request body: %v", err)
 			s3err.WriteErrorResponse(w, r, s3err.ErrInvalidRequest)