ipn/ipnlocal: use lockAndGetUnlock in Start

This removes one of the Lock,Unlock,Lock,Unlock at least in
the Start function. Still has 3 more of these.

Updates #11649

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -316,6 +316,7 @@ func TestStateMachine(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
+	b.DisablePortMapperForTest()
 
 	var cc, previousCC *mockControl
 	b.SetControlClientGetterForTesting(func(opts controlclient.Options) (controlclient.Client, error) {