freebsd: ignore IPv6 for now

FreeBSD tun devices don't work with the way we implement IPv6
https://github.com/tailscale/tailscale/issues/1307

At least for now, remove any IPv6 addresses from the netmap.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

wgengine/router/router.go

@@ -54,6 +54,8 @@ func Cleanup(logf logger.Logf, interfaceName string) {
 	cleanup(logf, interfaceName)
 }
 
+//go:generate go run tailscale.com/cmd/cloner -type=Config -output=config_clone.go
+
 // Config is the subset of Tailscale configuration that is relevant to
 // the OS's network stack.
 type Config struct {