net: dsa: use struct_size() in devm_kzalloc()

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:

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>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -767,11 +767,10 @@ static int dsa_switch_probe(struct dsa_switch *ds)
 
 struct dsa_switch *dsa_switch_alloc(struct device *dev, size_t n)
 {
-	size_t size = sizeof(struct dsa_switch) + n * sizeof(struct dsa_port);
 	struct dsa_switch *ds;
 	int i;
 
-	ds = devm_kzalloc(dev, size, GFP_KERNEL);
+	ds = devm_kzalloc(dev, struct_size(ds, ports, n), GFP_KERNEL);
 	if (!ds)
 		return NULL;