net: dsa: sja1105: Use struct_size() 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.

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

drivers/net/dsa/sja1105/sja1105_tas.c

@@ -475,8 +475,7 @@ bool sja1105_gating_check_conflicts(struct sja1105_private *priv, int port,
 	if (list_empty(&gating_cfg->entries))
 		return false;
 
-	dummy = kzalloc(sizeof(struct tc_taprio_sched_entry) * num_entries +
-			sizeof(struct tc_taprio_qopt_offload), GFP_KERNEL);
+	dummy = kzalloc(struct_size(dummy, entries, num_entries), GFP_KERNEL);
 	if (!dummy) {
 		NL_SET_ERR_MSG_MOD(extack, "Failed to allocate memory");
 		return true;