Staging: batman-adv: Update mtu of bat device by changing mtu of slave device

We must reduce our own mtu when we reduce the mtu of any device we use
to transfer our packets. Otherwise we may accept to large packets which
gets dropped by the actual device.

Reported-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>