fix: return context error from context reader (#9507)

cmd/bucket-handlers.go

@@ -628,7 +628,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 	bucket := mux.Vars(r)["bucket"]
 
 	// To detect if the client has disconnected.
-	r.Body = &detectDisconnect{r.Body, r.Context().Done()}
+	r.Body = &contextReader{r.Body, r.Context()}
 
 	// Require Content-Length to be set in the request
 	size := r.ContentLength