wgengine/router,ipn/ipnlocal: add MTU field to router config

The MacOS client can't set the MTU when creating the tun due to lack
of permissions, so add it to the router config and have MacOS set it
in the callback using a method that it does have permissions for.

Updates #8219

Signed-off-by: Val <valerie@tailscale.com>

wgengine/router/callback.go

@@ -24,9 +24,16 @@ type CallbackRouter struct {
 	// will return ErrGetBaseConfigNotSupported.
 	GetBaseConfigFunc func() (dns.OSConfig, error)
 
-	mu   sync.Mutex    // protects all the following
-	rcfg *Config       // last applied router config
-	dcfg *dns.OSConfig // last applied DNS config
+	// InitialMTU is the MTU the tun should be initialized with.
+	// Zero means don't change the MTU from the default. This MTU
+	// is applied only once, shortly after the TUN is created, and
+	// ignored thereafter.
+	InitialMTU uint32
+
+	mu        sync.Mutex    // protects all the following
+	didSetMTU bool          // if we set the MTU already
+	rcfg      *Config       // last applied router config
+	dcfg      *dns.OSConfig // last applied DNS config
 }
 
 // Up implements Router.
@@ -41,6 +48,10 @@ func (r *CallbackRouter) Set(rcfg *Config) error {
 	if r.rcfg.Equal(rcfg) {
 		return nil
 	}
+	if r.didSetMTU == false {
+		r.didSetMTU = true
+		rcfg.NewMTU = int(r.InitialMTU)
+	}
 	r.rcfg = rcfg
 	return r.SetBoth(r.rcfg, r.dcfg)
 }