ipn/ipnlocal: fix data race

We can't access b.netMap without holding b.mu.
We already grabbed it earlier in the function with the lock held.

Introduced in Nov 2020 in 7ea809897d.
Discovered during stress testing.
Apparently it's a pretty rare?

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
(cherry picked from commit f11a8928a6)

ipn/ipnlocal/local.go

@@ -2213,7 +2213,7 @@ func (b *LocalBackend) enterState(newState ipn.State) {
 		b.e.RequestStatus()
 	case ipn.Running:
 		var addrs []string
-		for _, addr := range b.netMap.Addresses {
+		for _, addr := range netMap.Addresses {
 			addrs = append(addrs, addr.IP().String())
 		}
 		systemd.Status("Connected; %s; %s", activeLogin, strings.Join(addrs, " "))