batman-adv: Fix hang up with small MTU hard-interface

The system hangs up when batman-adv soft-interface is created on
hard-interface with small MTU.  For example, the following commands
create batman-adv soft-interface on dummy interface with zero MTU:

  # ip link add name dummy0 type dummy
  # ip link set mtu 0 dev dummy0
  # ip link set up dev dummy0
  # ip link add name bat0 type batadv
  # ip link set dev dummy0 master bat0

These commands cause the system hang up with the following messages:

  [   90.578925][ T6689] batman_adv: bat0: Adding interface: dummy0
  [   90.580884][ T6689] batman_adv: bat0: The MTU of interface dummy0 is too small (0) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1560 would solve the problem.
  [   90.586264][ T6689] batman_adv: bat0: Interface activated: dummy0
  [   90.590061][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)
  [   90.595517][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)
  [   90.598499][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)

This patch fixes this issue by returning error when enabling
hard-interface with small MTU size.

Fixes: c6c8fea2 ("net: Add batman-adv meshing protocol")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/hard-interface.c

@@ -10,6 +10,7 @@
 #include <linux/atomic.h>
 #include <linux/byteorder/generic.h>
 #include <linux/container_of.h>
+#include <linux/errno.h>
 #include <linux/gfp.h>
 #include <linux/if.h>
 #include <linux/if_arp.h>
@@ -700,6 +701,9 @@ int batadv_hardif_enable_interface(struct batadv_hard_iface *hard_iface,
 	int max_header_len = batadv_max_header_len();
 	int ret;
 
+	if (hard_iface->net_dev->mtu < ETH_MIN_MTU + max_header_len)
+		return -EINVAL;
+
 	if (hard_iface->if_status != BATADV_IF_NOT_IN_USE)
 		goto out;