batman-adv: fix VLAN priority offset

batman-adv: fix VLAN priority offset

The batadv_skb_set_priority() receives an SKB with the inner ethernet
header at position "offset". When it tries to extract the IPv4 and IPv6
header, it needs to skip the ethernet header to get access to the IP
header.

But for VLAN header, it performs the access with the struct vlan_ethhdr.
This struct contains both both the ethernet header and the VLAN header. It
is therefore incorrect to skip over the whole vlan_ethhdr size to get
access to the vlan_ethhdr.

Cc: stable@vger.kernel.org
Fixes: c54f38c9aa22 ("batman-adv: set skb priority according to content")
Signed-off-by: Sven Eckelmann <sven@narfation.org>