usb: atm: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worse scenario, could lead to heap overflows.

Link: https://github.com/KSPP/linux/issues/160
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/20210928233935.GA299525@embeddedor
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/atm/usbatm.c

···
 	int error = -ENOMEM;
 	int i, length;
 	unsigned int maxpacket, num_packets;
+	size_t size;
 
 	/* instance init */
-	instance = kzalloc(sizeof(*instance) + sizeof(struct urb *) * (num_rcv_urbs + num_snd_urbs), GFP_KERNEL);
+	size = struct_size(instance, urbs, num_rcv_urbs + num_snd_urbs);
+	instance = kzalloc(size, GFP_KERNEL);
 	if (!instance)
 		return -ENOMEM;