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

The skb must be released in the receive handler since b91a2543
("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: 9e794b6b ("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>