cxgb4: 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.

This code was detected with the help of Coccinelle and, audited and
fixed manually.

Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/sge.c

@@ -2431,7 +2431,7 @@ int cxgb4_ethofld_send_flowc(struct net_device *dev, u32 eotid, u32 tc)
 	struct sk_buff *skb;
 	int ret = 0;
 
-	len = sizeof(*flowc) + sizeof(struct fw_flowc_mnemval) * nparams;
+	len = struct_size(flowc, mnemval, nparams);
 	len16 = DIV_ROUND_UP(len, 16);
 
 	entry = cxgb4_lookup_eotid(&adap->tids, eotid);