batman-adv: clear current gateway during teardown

In the Linux kernel, the following vulnerability has been resolved: batman-adv: clear current gateway during teardown batadv_gw_node_free() removes the gateway list entries during mesh teardown, but it does not clear the currently selected gateway. This leaves stale gateway state behind across cleanup and can break a later mesh recreation. Clear bat_priv->gw.curr_gw before walking the gateway list so the selected gateway reference is dropped as part of teardown.