net/switchdev: use struct_size over open coded arithmetic

Replace zero-length array with flexible-array member and make use
of the struct_size() helper in kmalloc(). For example:

struct switchdev_deferred_item {
    ...
    unsigned long data[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi (CGEL ZTE) <chi.minghao@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/switchdev/switchdev.c

@@ -85,7 +85,7 @@ static int switchdev_deferred_enqueue(struct net_device *dev,
 {
 	struct switchdev_deferred_item *dfitem;
 
-	dfitem = kmalloc(sizeof(*dfitem) + data_len, GFP_ATOMIC);
+	dfitem = kmalloc(struct_size(dfitem, data, data_len), GFP_ATOMIC);
 	if (!dfitem)
 		return -ENOMEM;
 	dfitem->dev = dev;