batman-adv: fix batman-adv header overhead calculation

Batman-adv prepends a full ethernet header in addition to its own
header. This has to be reflected in the MTU calculation, especially
since the value is used to set dev->hard_header_len.

Introduced by 411d6ed9


("batman-adv: consider network coding overhead when calculating required mtu")

Reported-by: cmsv <cmsv@wirelesspt.net>
Reported-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>