rdma: authenticate RC routes through the terminal error path

The RC auth adapter returned signature-verification errors to
Fiber, so the production S3 error handler collapsed them into a
generic 500 response; authentication failures lost their real
status and code the same way route failures did before the
terminal serializer. The adapter now sends verification errors
through the shared serializer as well.

cmd/vgwrdma/main.go

@@ -1162,7 +1162,7 @@ func runGateway(ctx context.Context, be backend.Backend) error {
 		// chain as usual.
 		rcAuth := func(ctx fiber.Ctx) error {
 			if err := rcVerify(ctx); err != nil {
-				return err
+				return rcroutes.WriteRouteError(ctx, err)
 			}
 			return ctx.Next()
 		}