wireguard backend: avoid error message if route already exists

While processing node add events, flannel-wg routes are added irrespective of whether they already exist. This causes an error message for each wg peer. This might be misleading since there is actually no problem. By add or replacing the route, this can be avoided.

Fixes flannel-io#1963

pkg/backend/wireguard/device.go

@@ -233,7 +233,7 @@ func (dev *wgDevice) addRoute(dst *net.IPNet) error {
 		Dst:       dst,
 	}
 
-	err := netlink.RouteAdd(&route)
+	err := netlink.RouteReplace(&route)
 	if err != nil {
 		return fmt.Errorf("failed to add route %s: %w", dev.attrs.name, err)
 	}