mpls_iptunnel: 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[];
};

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

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));

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/mpls/mpls_iptunnel.c

@@ -183,8 +183,8 @@ static int mpls_build_state(struct nlattr *nla,
 			   &n_labels, NULL, extack))
 		return -EINVAL;
 
-	newts = lwtunnel_state_alloc(sizeof(*tun_encap_info) +
-				     n_labels * sizeof(u32));
+	newts = lwtunnel_state_alloc(struct_size(tun_encap_info, label,
+						 n_labels));
 	if (!newts)
 		return -ENOMEM;