batman-adv: Check skb size before using encapsulated ETH+VLAN header

[ Upstream commit c78296665c3d81f040117432ab9e1cb125521b0c ]

The encapsulated ethernet and VLAN header may be outside the received
ethernet frame. Thus the skb buffer size has to be checked before it can be
parsed to find out if it encapsulates another batman-adv packet.

Fixes: 42019357 ("batman-adv: softif bridge loop avoidance")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>