batman-adv: Remove variable deprecated by skb_put_data

skb_put_data makes it unnecessary to store the skb_put return value to copy
some data to the packet. The returned pointer of skb_put_data should
therefore not stored by functions which previously only used it to copy
some data.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>