usb: atm: Use size_add() in call to struct_size()

If, for any reason, the open-coded arithmetic causes a wraparound,
the protection that `struct_size()` adds against potential integer
overflows is defeated. Fix this by hardening call to `struct_size()`
with `size_add()`.

Fixes: b626871a ("usb: atm: Use struct_size() helper")
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZQSuboEIhvATAdxN@workSigned-off-by: Kees Cook <keescook@chromium.org>

drivers/usb/atm/usbatm.c

@@ -1018,7 +1018,8 @@ int usbatm_usb_probe(struct usb_interface *intf, const struct usb_device_id *id,
 	size_t size;
 
 	/* instance init */
-	size = struct_size(instance, urbs, num_rcv_urbs + num_snd_urbs);
+	size = struct_size(instance, urbs,
+			   size_add(num_rcv_urbs, num_snd_urbs));
 	instance = kzalloc(size, GFP_KERNEL);
 	if (!instance)
 		return -ENOMEM;