batman-adv: Fix bat_ogm_iv best gw refcnt after netlink dump

A reference for the best gateway is taken when the list of gateways in the
mesh is sent via netlink. This is necessary to check whether the currently
dumped entry is the currently selected gateway or not. This information is
then transferred as flag BATADV_ATTR_FLAG_BEST.

After the comparison of the current entry is done,
batadv_iv_gw_dump_entry() has to decrease the reference counter again.
Otherwise the reference will be held and thus prevents a proper shutdown of
the batman-adv interfaces (and some of the interfaces enslaved in it).

Fixes: efb766af ("batman-adv: add B.A.T.M.A.N. IV bat_gw_dump implementations")
Reported-by: Andreas Ziegler <dev@andreas-ziegler.de>
Tested-by: Andreas Ziegler <dev@andreas-ziegler.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bat_iv_ogm.c

@@ -2732,7 +2732,7 @@ static int batadv_iv_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
 {
 	struct batadv_neigh_ifinfo *router_ifinfo = NULL;
 	struct batadv_neigh_node *router;
-	struct batadv_gw_node *curr_gw;
+	struct batadv_gw_node *curr_gw = NULL;
 	int ret = 0;
 	void *hdr;
 
@@ -2780,6 +2780,8 @@ static int batadv_iv_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
 	ret = 0;
 
 out:
+	if (curr_gw)
+		batadv_gw_node_put(curr_gw);
 	if (router_ifinfo)
 		batadv_neigh_ifinfo_put(router_ifinfo);
 	if (router)