High CPU usage when the network is disconnected

### What is the issue?

When the network is disconnected, tailscaled consumes a large amount of CPU, but it immediately recovers when the network is reconnected.

### Steps to reproduce

- 1、When there are peer nodes in the local network
- 2、Disconnect the current network connection, such as turning off Wi-Fi
- 3、Check the CPU usage and find that tailscaled is consuming a large amount of CPU.

### Are there any recent changes that introduced the issue?

I have noticed this issue for a long time, to be more precise, after the fix of #5879.

### OS

macOS

### OS version

13.4.1 (c) (22F770820d)

### Tailscale version

1.44.0-ERR-BuildInfo

### Other software

- 1、[Surge](https://nssurge.com/)

### Bug report

BUG-c14296d9810c1b534d7f874b3cb34ca6c8eb032c3f065ff04d173dc0a04d2894-20230717091416Z-065659205319fc4a