net: thunderx: Use struct_size() helper in kmalloc()

Make use of the new struct_size() helper instead of the offsetof() idiom.
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/cavium/thunder/nicvf_main.c

@@ -2065,8 +2065,8 @@ static void nicvf_set_rx_mode(struct net_device *netdev)
 			mode |= BGX_XCAST_MCAST_FILTER;
 			/* here we need to copy mc addrs */
 			if (netdev_mc_count(netdev)) {
-				mc_list = kmalloc(offsetof(typeof(*mc_list),
-							   mc[netdev_mc_count(netdev)]),
+				mc_list = kmalloc(struct_size(mc_list, mc,
+							      netdev_mc_count(netdev)),
 						  GFP_ATOMIC);
 				if (unlikely(!mc_list))
 					return;