batman-adv: clear current gateway during teardown

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.

Fixes: 2265c1410864 ("batman-adv: gateway election code refactoring")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Ruijie Li <ruijieli51@gmail.com>
Signed-off-by: Zhanpeng Li <lzhanpeng2025@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Sven Eckelmann <sven@narfation.org>