nfp: flower: cmsg: 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;
    void *entry[];
};

size = sizeof(struct foo) + count * sizeof(void *);
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:

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

Notice that, in this case, variable size is not necessary, hence
it is removed.

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

drivers/net/ethernet/netronome/nfp/flower/cmsg.c

@@ -45,11 +45,9 @@ nfp_flower_cmsg_mac_repr_start(struct nfp_app *app, unsigned int num_ports)
 {
 	struct nfp_flower_cmsg_mac_repr *msg;
 	struct sk_buff *skb;
-	unsigned int size;
 
-	size = sizeof(*msg) + num_ports * sizeof(msg->ports[0]);
-	skb = nfp_flower_cmsg_alloc(app, size, NFP_FLOWER_CMSG_TYPE_MAC_REPR,
-				    GFP_KERNEL);
+	skb = nfp_flower_cmsg_alloc(app, struct_size(msg, ports, num_ports),
+				    NFP_FLOWER_CMSG_TYPE_MAC_REPR, GFP_KERNEL);
 	if (!skb)
 		return NULL;