Bluetooth: a2mp: 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);
instance = alloc(size, GFP_KERNEL)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/a2mp.c

@@ -174,7 +174,7 @@ static int a2mp_discover_req(struct amp_mgr *mgr, struct sk_buff *skb,
 			num_ctrl++;
 	}
 
-	len = num_ctrl * sizeof(struct a2mp_cl) + sizeof(*rsp);
+	len = struct_size(rsp, cl, num_ctrl);
 	rsp = kmalloc(len, GFP_ATOMIC);
 	if (!rsp) {
 		read_unlock(&hci_dev_list_lock);