net: stmmac: selftests: Use struct_size() helper in kzalloc()

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.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c

@@ -1094,7 +1094,7 @@ static int stmmac_test_rxp(struct stmmac_priv *priv)
 	if (!priv->dma_cap.frpsel)
 		return -EOPNOTSUPP;
 
-	sel = kzalloc(sizeof(*sel) + nk * sizeof(struct tc_u32_key), GFP_KERNEL);
+	sel = kzalloc(struct_size(sel, keys, nk), GFP_KERNEL);
 	if (!sel)
 		return -ENOMEM;