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

@@ -668,12 +668,14 @@ 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()
@@ -682,11 +684,8 @@ func (c *Auto) Shutdown() {
 		w <- false
 	}
 	c.unpauseWaiters = nil
-	}
 	c.mu.Unlock()
 
-	c.logf("client.Shutdown")
-	if !closed {
 	c.unregisterHealthWatch()
 	<-c.authDone
 	<-c.mapDone
@@ -698,7 +697,6 @@ func (c *Auto) Shutdown() {
 	defer cancel()
 	c.observerQueue.Wait(ctx)
 	c.logf("Client.Shutdown done.")
-	}
 }
 
 // NodePublicKey returns the node public key currently in use. This is