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

The module prints a warning when the MTU on the hard interface is too
small to transfer payload traffic without fragmentation. The required
MTU is calculated based on the encapsulation header size. If network
coding is compild into the module its header size is taken into
account as well.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
diff --git a/net/batman-adv/main.h b/net/batman-adv/main.h
index 6a74a42..54c13d5 100644
--- a/net/batman-adv/main.h
+++ b/net/batman-adv/main.h
@@ -191,6 +191,7 @@
 int batadv_is_my_mac(struct batadv_priv *bat_priv, const uint8_t *addr);
 struct batadv_hard_iface *
 batadv_seq_print_text_primary_if_get(struct seq_file *seq);
+int batadv_max_header_len(void);
 void batadv_skb_set_priority(struct sk_buff *skb, int offset);
 int batadv_batman_skb_recv(struct sk_buff *skb, struct net_device *dev,
 			   struct packet_type *ptype,