netfilter: xt_hashlimit: use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace code of the following form:

sizeof(struct xt_hashlimit_htable) + sizeof(struct hlist_head) * size

with:

struct_size(hinfo, hash, size)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_hashlimit.c

@@ -288,8 +288,7 @@ static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
 			size = 16;
 	}
 	/* FIXME: don't use vmalloc() here or anywhere else -HW */
-	hinfo = vmalloc(sizeof(struct xt_hashlimit_htable) +
-	                sizeof(struct hlist_head) * size);
+	hinfo = vmalloc(struct_size(hinfo, hash, size));
 	if (hinfo == NULL)
 		return -ENOMEM;
 	*out_hinfo = hinfo;