net/sched: cls_u32: 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.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -796,9 +796,7 @@ static struct tc_u_knode *u32_init_knode(struct net *net, struct tcf_proto *tp,
 	struct tc_u32_sel *s = &n->sel;
 	struct tc_u_knode *new;
 
-	new = kzalloc(sizeof(*n) + s->nkeys*sizeof(struct tc_u32_key),
-		      GFP_KERNEL);
-
+	new = kzalloc(struct_size(new, sel.keys, s->nkeys), GFP_KERNEL);
 	if (!new)
 		return NULL;