batman-adv: Fix incorrect offset in batadv_tt_tvlv_ogm_handler_v1()

Since commit 4436df4 ("batman-adv: Add flex array to struct
batadv_tvlv_tt_data"), the introduction of batadv_tvlv_tt_data's flex
array member in batadv_tt_tvlv_ogm_handler_v1() put tt_changes at
invalid offset. Those TT changes are supposed to be filled from the end
of batadv_tvlv_tt_data structure (including vlan_data flexible array),
but only the flex array size is taken into account missing completely
the size of the fixed part of the structure itself.

Fix the tt_change offset computation by using struct_size() instead of
flex_array_size() so both flex array member and its container structure
sizes are taken into account.

Cc: stable@vger.kernel.org
Fixes: 4436df4 ("batman-adv: Add flex array to struct batadv_tvlv_tt_data")
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

Author: repk

net/batman-adv/translation-table.c

@@ -3959,23 +3959,21 @@ static void batadv_tt_tvlv_ogm_handler_v1(struct batadv_priv *bat_priv,
 	struct batadv_tvlv_tt_change *tt_change;
 	struct batadv_tvlv_tt_data *tt_data;
 	u16 num_entries, num_vlan;
-	size_t flex_size;
+	size_t tt_data_sz;
 
 	if (tvlv_value_len < sizeof(*tt_data))
 		return;
 
 	tt_data = tvlv_value;
-	tvlv_value_len -= sizeof(*tt_data);
-
 	num_vlan = ntohs(tt_data->num_vlan);
 
-	flex_size = flex_array_size(tt_data, vlan_data, num_vlan);
-	if (tvlv_value_len < flex_size)
+	tt_data_sz = struct_size(tt_data, vlan_data, num_vlan);
+	if (tvlv_value_len < tt_data_sz)
 		return;
 
 	tt_change = (struct batadv_tvlv_tt_change *)((void *)tt_data
-						     + flex_size);
-	tvlv_value_len -= flex_size;
+						     + tt_data_sz);
+	tvlv_value_len -= tt_data_sz;
 
 	num_entries = batadv_tt_entries(tvlv_value_len);