batman-adv: fix skb deref after free

batadv_send_skb_to_orig() calls dev_queue_xmit() so we can't use skb->len.

Fixes: 95332477

 ("batman-adv: network coding - buffer unicast packets before forward")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>