batman-adv: fix memory leak when dropping packet from other gateway

The skb must be released in the receive handler since b91a2543b4c1
("batman-adv: Consume skb in receive handlers"). Just returning NET_RX_DROP
will no longer automatically free the memory. This results in memory leaks
when unicast packets from other backbones must be dropped because they
share a common backbone.

Fixes: 9e794b6bf4a2 ("batman-adv: drop unicast packets from other backbone gw")
Signed-off-by: Andreas Pape <apape@phoenixcontact.com>
[sven@narfation.org: adjust commit message]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/routing.c

@@ -987,7 +987,7 @@ int batadv_recv_unicast_packet(struct sk_buff *skb,
 				batadv_dbg(BATADV_DBG_BLA, bat_priv,
 					   "recv_unicast_packet(): Dropped unicast pkt received from another backbone gw %pM.\n",
 					   orig_addr_gw);
-				return NET_RX_DROP;
+				goto free_skb;
 			}
 		}