ipn/ipnlocal: fix another regression from link monitoring refactor

Prior to e3df29d488, the Engine.SetLinkChangeCallback fired
immediately, even if there was no change. The ipnlocal code apparently
depended on that, and it broke integration tests (which live in
another repo). So mimic the old behavior and call the ipnlocal
callback immediately at init.

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

ipn/ipnlocal/local.go

@@ -139,14 +139,19 @@ func NewLocalBackend(logf logger.Logf, logid string, store ipn.StateStore, e wge
 		portpoll:       portpoll,
 		gotPortPollRes: make(chan struct{}),
 	}
-	b.unregisterLinkMon = e.GetLinkMonitor().RegisterChangeCallback(b.linkChange)
 	b.statusChanged = sync.NewCond(&b.statusLock)
 
+	linkMon := e.GetLinkMonitor()
+	// Call our linkChange code once with the current state, and
+	// then also whenever it changes:
+	b.linkChange(false, linkMon.InterfaceState())
+	b.unregisterLinkMon = linkMon.RegisterChangeCallback(b.linkChange)
+
 	return b, nil
 }
 
-// linkChange is called (in a new goroutine) by wgengine when its link monitor
+// linkChange is our link monitor callback, called whenever the network changes.
-// detects a network change.
+// major is whether ifst is different than earlier.
 func (b *LocalBackend) linkChange(major bool, ifst *interfaces.State) {
 	b.mu.Lock()
 	defer b.mu.Unlock()