taprio: 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. Also, remove unnecessary
variable _size_.

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>

net/sched/sch_taprio.c

@@ -1108,11 +1108,10 @@ static void setup_txtime(struct taprio_sched *q,
 
 static struct tc_taprio_qopt_offload *taprio_offload_alloc(int num_entries)
 {
-	size_t size = sizeof(struct tc_taprio_sched_entry) * num_entries +
-		      sizeof(struct __tc_taprio_qopt_offload);
 	struct __tc_taprio_qopt_offload *__offload;
 
-	__offload = kzalloc(size, GFP_KERNEL);
+	__offload = kzalloc(struct_size(__offload, offload.entries, num_entries),
+			    GFP_KERNEL);
 	if (!__offload)
 		return NULL;