net/portmapper: ignore IGD SSDP responses from !defaultgw

Now that we multicast the SSDP query, we can get IGD offers from
devices other than the current device's default gateway. We don't want
to accidentally bind ourselves to those.

Updates #3197

Signed-off-by: David Anderson <danderson@tailscale.com>

net/portmapper/portmapper.go

@@ -752,10 +752,14 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 			}
 			return res, err
 		}
+		ip, ok := netaddr.FromStdIP(addr.(*net.UDPAddr).IP)
+		if !ok {
+			continue
+		}
 		port := uint16(addr.(*net.UDPAddr).Port)
 		switch port {
 		case c.upnpPort():
-			if mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
+			if ip == gw && mem.Contains(mem.B(buf[:n]), mem.S(":InternetGatewayDevice:")) {
 				meta, err := parseUPnPDiscoResponse(buf[:n])
 				if err != nil {
 					c.logf("unrecognized UPnP discovery response; ignoring")