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/soft-interface.c b/net/batman-adv/soft-interface.c
index 87e7e4e..15c7237 100644
--- a/net/batman-adv/soft-interface.c
+++ b/net/batman-adv/soft-interface.c
@@ -626,7 +626,7 @@
 	 */
 	dev->mtu = ETH_DATA_LEN;
 	/* reserve more space in the skbuff for our header */
-	dev->hard_header_len = BATADV_HEADER_LEN;
+	dev->hard_header_len = batadv_max_header_len();
 
 	/* generate random address */
 	eth_hw_addr_random(dev);