batman-adv: fix memory leak in in batadv_dat_put_dhcp

batadv_dat_put_dhcp is creating a new ARP packet via
batadv_dat_arp_create_reply and tries to forward it via
batadv_dat_send_data to different peers in the DHT. The original skb is not
consumed by batadv_dat_send_data and thus has to be consumed by the caller.

Fixes: b61ec31c ("batman-adv: Snoop DHCPACKs for DAT")
Signed-off-by: Martin Weinelt <martin@linuxlounge.net>
[sven@narfation.org: add commit message]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>