Bluetooth: mgmt: 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 mgmt_rp_get_connections {
	...
        struct mgmt_addr_info addr[0];
} __packed;

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

So, replace the following form:

sizeof(*rp) + (i * sizeof(struct mgmt_addr_info));

with:

struct_size(rp, addr, i)

Also, notice that, in this case, variable rp_len 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>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/mgmt.c

@@ -2588,7 +2588,6 @@ static int get_connections(struct sock *sk, struct hci_dev *hdev, void *data,
 {
 	struct mgmt_rp_get_connections *rp;
 	struct hci_conn *c;
-	size_t rp_len;
 	int err;
 	u16 i;
 
@@ -2608,8 +2607,7 @@ static int get_connections(struct sock *sk, struct hci_dev *hdev, void *data,
 			i++;
 	}
 
-	rp_len = sizeof(*rp) + (i * sizeof(struct mgmt_addr_info));
-	rp = kmalloc(rp_len, GFP_KERNEL);
+	rp = kmalloc(struct_size(rp, addr, i), GFP_KERNEL);
 	if (!rp) {
 		err = -ENOMEM;
 		goto unlock;
@@ -2629,10 +2627,8 @@ static int get_connections(struct sock *sk, struct hci_dev *hdev, void *data,
 	rp->conn_count = cpu_to_le16(i);
 
 	/* Recalculate length in case of filtered SCO connections, etc */
-	rp_len = sizeof(*rp) + (i * sizeof(struct mgmt_addr_info));
-
 	err = mgmt_cmd_complete(sk, hdev->id, MGMT_OP_GET_CONNECTIONS, 0, rp,
-				rp_len);
+				struct_size(rp, addr, i));
 
 	kfree(rp);