batman-adv: fix skb leak in batadv_dat_snoop_incoming_arp_reply()

The callers of batadv_dat_snoop_incoming_arp_reply() assume the skb has been
freed when it returns true; fix this by calling kfree_skb before returning as
it is done in batadv_dat_snoop_incoming_arp_request().

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>