nfp: flower: 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 nfp_tun_active_tuns {
	...
        struct route_ip_info {
                __be32 ipv4;
                __be32 egress_port;
                __be32 extra[2];
        } tun_info[];
};

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(struct nfp_tun_active_tuns) + sizeof(struct route_ip_info) * count

with:

struct_size(payload, tun_info, count)

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/tunnel_conf.c

@@ -162,8 +162,7 @@ void nfp_tunnel_keep_alive(struct nfp_app *app, struct sk_buff *skb)
 	}
 
 	pay_len = nfp_flower_cmsg_get_data_len(skb);
-	if (pay_len != sizeof(struct nfp_tun_active_tuns) +
-	    sizeof(struct route_ip_info) * count) {
+	if (pay_len != struct_size(payload, tun_info, count)) {
 		nfp_flower_cmsg_warn(app, "Corruption in tunnel keep-alive message.\n");
 		return;
 	}