bridge: 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[];
};

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

size = 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/bridge/br_multicast.c

@@ -1018,8 +1018,7 @@ static int br_ip6_multicast_mld2_report(struct net_bridge *br,
 		if (!nsrcs)
 			return -EINVAL;
 
-		grec_len = sizeof(*grec) +
-			   sizeof(struct in6_addr) * ntohs(*nsrcs);
+		grec_len = struct_size(grec, grec_src, ntohs(*nsrcs));
 
 		if (!ipv6_mc_may_pull(skb, len + grec_len))
 			return -EINVAL;