ipn/ipnlocal: fix deadlock in resetControlClientLocked

resetControlClientLocked is called while b.mu was held and
would call cc.Shutdown which would wait for the observer queue
to drain.
However, there may be active callbacks from cc already waiting for
b.mu resulting in a deadlock.

This makes it so that resetControlClientLocked does not call
Shutdown, and instead just returns the value.
It also makes it so that any status received from previous cc
are ignored.

Updates tailscale/corp#12827

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

control/controlclient/auto.go

@@ -595,7 +595,7 @@ func (c *Auto) sendStatus(who string, err error, url string, nm *netmap.NetworkM
 	// Launch a new goroutine to avoid blocking the caller while the observer
 	// does its thing, which may result in a call back into the client.
 	c.observerQueue.Add(func() {
-		c.observer.SetControlClientStatus(new)
+		c.observer.SetControlClientStatus(c, new)
 	})
 }
 
@@ -667,6 +667,7 @@ func (c *Auto) Shutdown() {
 	direct := c.direct
 	if !closed {
 		c.closed = true
+		c.observerQueue.shutdown()
 		c.cancelAuthCtxLocked()
 		c.cancelMapCtxLocked()
 		for _, w := range c.unpauseWaiters {