control/controlclient: early return and outdent some code

I found this too hard to read before.

This is pulled out of #12033 as it's unrelated cleanup in retrospect.

Updates #12028

Change-Id: I727c47e573217e3d1973c5b66a76748139cf79ee
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/auto.go

@@ -668,37 +668,35 @@ func (c *Auto) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 }
 
 func (c *Auto) Shutdown() {
-	c.logf("client.Shutdown()")
-
 	c.mu.Lock()
-	closed := c.closed
+	if c.closed {
+		c.mu.Unlock()
+		return
+	}
+	c.logf("client.Shutdown ...")
+
 	direct := c.direct
-	if !closed {
-		c.closed = true
-		c.observerQueue.Shutdown()
-		c.cancelAuthCtxLocked()
-		c.cancelMapCtxLocked()
-		for _, w := range c.unpauseWaiters {
-			w <- false
-		}
-		c.unpauseWaiters = nil
+	c.closed = true
+	c.observerQueue.Shutdown()
+	c.cancelAuthCtxLocked()
+	c.cancelMapCtxLocked()
+	for _, w := range c.unpauseWaiters {
+		w <- false
 	}
+	c.unpauseWaiters = nil
 	c.mu.Unlock()
 
-	c.logf("client.Shutdown")
-	if !closed {
-		c.unregisterHealthWatch()
-		<-c.authDone
-		<-c.mapDone
-		<-c.updateDone
-		if direct != nil {
-			direct.Close()
-		}
-		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-		defer cancel()
-		c.observerQueue.Wait(ctx)
-		c.logf("Client.Shutdown done.")
+	c.unregisterHealthWatch()
+	<-c.authDone
+	<-c.mapDone
+	<-c.updateDone
+	if direct != nil {
+		direct.Close()
 	}
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	c.observerQueue.Wait(ctx)
+	c.logf("Client.Shutdown done.")
 }
 
 // NodePublicKey returns the node public key currently in use. This is