net/netmon: log when the gateway/self IP changes

This logs that the gateway/self IP address has changed if one of the new
values differs.

Updates #8992

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0919424b68ad97fbe1204dd36317ed6f5915411f

net/netmon/netmon.go

@@ -174,8 +174,14 @@ func (m *Monitor) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
 		return m.gw, m.gwSelfIP, true
 	}
 	gw, myIP, ok = interfaces.LikelyHomeRouterIP()
+	changed := false
 	if ok {
-		m.gw, m.gwSelfIP, m.gwValid = gw, myIP, true
+		changed = m.gw != gw || m.gwSelfIP != myIP
+		m.gw, m.gwSelfIP = gw, myIP
+		m.gwValid = true
+	}
+	if changed {
+		m.logf("gateway and self IP changed: gw=%v self=%v", m.gw, m.gwSelfIP)
 	}
 	return gw, myIP, ok
 }