wgengine/router: avoid unncessary routing configuration changes

The iOS and macOS networking extension API only exposes a single setter
for the entire routing and DNS configuration, and does not appear to
do any kind of diffing or deltas when applying changes. This results
in spurious "network changed" errors in Chrome, even when the
`OneCGNATRoute` flag from df9ce972c7 is
used (because we're setting the same configuration repeatedly).

Since we already keep track of the current routing and DNS configuration
in CallbackRouter, use that to detect if they're actually changing, and
only invoke the platform setter if it's actually necessary.

Updates #3102

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

wgengine/router/router.go

@@ -7,6 +7,8 @@
 package router
 
 import (
+	"reflect"
+
 	"golang.zx2c4.com/wireguard/tun"
 	"inet.af/netaddr"
 	"tailscale.com/types/logger"
@@ -72,6 +74,16 @@ type Config struct {
 	NetfilterMode    preftype.NetfilterMode // how much to manage netfilter rules
 }
 
+func (a *Config) Equal(b *Config) bool {
+	if a == nil && b == nil {
+		return true
+	}
+	if (a == nil) != (b == nil) {
+		return false
+	}
+	return reflect.DeepEqual(a, b)
+}
+
 // shutdownConfig is a routing configuration that removes all router
 // state from the OS. It's the config used when callers pass in a nil
 // Config.