enetc: 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>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -1687,7 +1687,7 @@ int enetc_ioctl(struct net_device *ndev, struct ifreq *rq, int cmd)
 int enetc_alloc_msix(struct enetc_ndev_priv *priv)
 {
 	struct pci_dev *pdev = priv->si->pdev;
-	int size, v_tx_rings;
+	int v_tx_rings;
 	int i, n, err, nvec;
 
 	nvec = ENETC_BDR_INT_BASE_IDX + priv->bdr_int_num;
@@ -1702,15 +1702,13 @@ int enetc_alloc_msix(struct enetc_ndev_priv *priv)
 
 	/* # of tx rings per int vector */
 	v_tx_rings = priv->num_tx_rings / priv->bdr_int_num;
-	size = sizeof(struct enetc_int_vector) +
-	       sizeof(struct enetc_bdr) * v_tx_rings;
 
 	for (i = 0; i < priv->bdr_int_num; i++) {
 		struct enetc_int_vector *v;
 		struct enetc_bdr *bdr;
 		int j;
 
-		v = kzalloc(size, GFP_KERNEL);
+		v = kzalloc(struct_size(v, tx_ring, v_tx_rings), GFP_KERNEL);
 		if (!v) {
 			err = -ENOMEM;
 			goto fail;