flow_offload: use struct_size() in kzalloc()

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[];
};

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

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

instance = kzalloc(struct_size(instance, entry, count), 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/core/flow_offload.c

@@ -7,8 +7,7 @@ struct flow_rule *flow_rule_alloc(unsigned int num_actions)
 {
 	struct flow_rule *rule;
 
-	rule = kzalloc(sizeof(struct flow_rule) +
-		       sizeof(struct flow_action_entry) * num_actions,
+	rule = kzalloc(struct_size(rule, action.entries, num_actions),
 		       GFP_KERNEL);
 	if (!rule)
 		return NULL;