udp_tunnel: Use flex array to simplify code

'n_tables' is small, UDP_TUNNEL_NIC_MAX_TABLES	= 4 as a maximum. So there
is no real point to allocate the 'entries' pointers array with a dedicate
memory allocation.

Using a flexible array for struct udp_tunnel_nic->entries avoids the
overhead of an additional memory allocation.

This also saves an indirection when the array is accessed.

Finally, __counted_by() can be used for run-time bounds checking if
configured and supported by the compiler.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/4a096ba9cf981a588aa87235bb91e933ee162b3d.1695542544.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/udp_tunnel_nic.c

@@ -47,7 +47,7 @@ struct udp_tunnel_nic {
 
 	unsigned int n_tables;
 	unsigned long missed;
-	struct udp_tunnel_nic_table_entry **entries;
+	struct udp_tunnel_nic_table_entry *entries[] __counted_by(n_tables);
 };
 
 /* We ensure all work structs are done using driver state, but not the code.
@@ -725,16 +725,12 @@ udp_tunnel_nic_alloc(const struct udp_tunnel_nic_info *info,
 	struct udp_tunnel_nic *utn;
 	unsigned int i;
 
-	utn = kzalloc(sizeof(*utn), GFP_KERNEL);
+	utn = kzalloc(struct_size(utn, entries, n_tables), GFP_KERNEL);
 	if (!utn)
 		return NULL;
 	utn->n_tables = n_tables;
 	INIT_WORK(&utn->work, udp_tunnel_nic_device_sync_work);
 
-	utn->entries = kmalloc_array(n_tables, sizeof(void *), GFP_KERNEL);
-	if (!utn->entries)
-		goto err_free_utn;
-
 	for (i = 0; i < n_tables; i++) {
 		utn->entries[i] = kcalloc(info->tables[i].n_entries,
 					  sizeof(*utn->entries[i]), GFP_KERNEL);
@@ -747,8 +743,6 @@ udp_tunnel_nic_alloc(const struct udp_tunnel_nic_info *info,
 err_free_prev_entries:
 	while (i--)
 		kfree(utn->entries[i]);
-	kfree(utn->entries);
-err_free_utn:
 	kfree(utn);
 	return NULL;
 }
@@ -759,7 +753,6 @@ static void udp_tunnel_nic_free(struct udp_tunnel_nic *utn)
 
 	for (i = 0; i < utn->n_tables; i++)
 		kfree(utn->entries[i]);
-	kfree(utn->entries);
 	kfree(utn);
 }