net/sched: use struct_size() helper

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = alloc(size, GFP_KERNEL)

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

size = struct_size(instance, entry, count);
instance = alloc(size, GFP_KERNEL)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_pedit.c

@@ -406,7 +406,7 @@ static int tcf_pedit_dump(struct sk_buff *skb, struct tc_action *a,
 	struct tcf_t t;
 	int s;
 
-	s = sizeof(*opt) + p->tcfp_nkeys * sizeof(struct tc_pedit_key);
+	s = struct_size(opt, keys, p->tcfp_nkeys);
 
 	/* netlink spinlocks held above us - must use ATOMIC */
 	opt = kzalloc(s, GFP_ATOMIC);