i2c: mux: 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[];
};

instance = devm_kzalloc(dev, sizeof(struct foo) + count * sizeof(struct boo), GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = devm_kzalloc(dev, struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Peter Rosin <peda@axentia.se>

drivers/i2c/i2c-mux.c

@@ -243,8 +243,7 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
 {
 	struct i2c_mux_core *muxc;
 
-	muxc = devm_kzalloc(dev, sizeof(*muxc)
-			    + max_adapters * sizeof(muxc->adapter[0])
+	muxc = devm_kzalloc(dev, struct_size(muxc, adapter, max_adapters)
 			    + sizeof_priv, GFP_KERNEL);
 	if (!muxc)
 		return NULL;