batman-adv: tt: replace open-coded overflow check with helper

The commit 6043a632dd06 ("batman-adv: reject oversized global TT response
buffers") introduced an open-coded check to ensure that the allocated
buffer size can be stored in a u16. The check_add_overflow() helper can
perform the addition and overflow check in one step, so use that instead.

Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

Author: ecsv

net/batman-adv/translation-table.c

@@ -799,10 +799,10 @@ batadv_tt_prepare_tvlv_global_data(struct batadv_orig_node *orig_node,
 {
 	u16 num_vlan = 0;
 	u16 tvlv_len = 0;
-	unsigned int change_offset;
 	struct batadv_tvlv_tt_vlan_data *tt_vlan;
 	struct batadv_orig_node_vlan *vlan;
 	u16 total_entries = 0;
+	size_t change_offset;
 	u8 *tt_change_ptr;
 	int vlan_entries;
 	u16 sum_entries;
@@ -826,14 +826,11 @@ batadv_tt_prepare_tvlv_global_data(struct batadv_orig_node *orig_node,
 	if (*tt_len < 0)
 		*tt_len = batadv_tt_len(total_entries);
 
-	if (change_offset > U16_MAX || *tt_len > U16_MAX - change_offset) {
+	if (check_add_overflow(*tt_len, change_offset, &tvlv_len)) {
 		*tt_len = 0;
 		goto out;
 	}
 
-	tvlv_len = *tt_len;
-	tvlv_len += change_offset;
-
 	*tt_data = kmalloc(tvlv_len, GFP_ATOMIC);
 	if (!*tt_data) {
 		*tt_len = 0;