ipn/ipnlocal: derive the OS routes from the route manager

routerConfigLocked previously computed router.Config.Routes with
peerRoutes, a from-scratch pass over cfg.Peers on every reconfig.
The route manager already maintains the same set incrementally (ULA
and CGNAT coarsening included) and updateRouteManagerPrefs runs
earlier in authReconfig, so its OS route set is current by the time
the router config is built. Read it from there instead, and delete
peerRoutes and its tests; the routemanager package tests cover the
same behavior. This removes a consumer of cfg.Peers, which is on
its way out.

Updates #12542

Change-Id: I4a5b7a63d530e3fe1b70f0faf3f49def6a10be2e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/node_backend.go

@@ -861,6 +861,17 @@ func (nb *nodeBackend) updateRouteManagerPrefs(p routePrefs) (changedAllowedIPs
 	return res.AllowedIPs
 }
 
+// osRoutes returns the sorted set of prefixes that the route manager
+// wants programmed into the OS routing table.
+func (nb *nodeBackend) osRoutes() []netip.Prefix {
+	var routes []netip.Prefix
+	for pfx := range nb.routeMgr.OSRoutes().All() {
+		routes = append(routes, pfx)
+	}
+	tsaddr.SortPrefixes(routes)
+	return routes
+}
+
 // setPacketFilter stores the live packet filter rules and parsed
 // matches. It does not touch the frozen netMap. nb.mu is acquired by
 // this method.