cmd/tailscaled,net/tstun: fix data race on start-up in TUN mode

Fixes #7894

Change-Id: Ice3f8019405714dd69d02bc07694f3872bb598b8

Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -540,6 +540,10 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 	}
 	sys.Set(store)
 
+	if w, ok := sys.Tun.GetOK(); ok {
+		w.Start()
+	}
+
 	lb, err := ipnlocal.NewLocalBackend(logf, logID, sys, opts.LoginFlags)
 	if err != nil {
 		return nil, fmt.Errorf("ipnlocal.NewLocalBackend: %w", err)