Bluetooth: mgmt: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, change the following form:

sizeof(*rp) + (sizeof(rp->entry[0]) * count);

to :

struct_size(rp, entry, count)

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

@@ -474,7 +474,6 @@ static int read_ext_index_list(struct sock *sk, struct hci_dev *hdev,
 {
 	struct mgmt_rp_read_ext_index_list *rp;
 	struct hci_dev *d;
-	size_t rp_len;
 	u16 count;
 	int err;
 
@@ -488,8 +487,7 @@ static int read_ext_index_list(struct sock *sk, struct hci_dev *hdev,
 			count++;
 	}
 
-	rp_len = sizeof(*rp) + (sizeof(rp->entry[0]) * count);
-	rp = kmalloc(rp_len, GFP_ATOMIC);
+	rp = kmalloc(struct_size(rp, entry, count), GFP_ATOMIC);
 	if (!rp) {
 		read_unlock(&hci_dev_list_lock);
 		return -ENOMEM;
@@ -525,7 +523,6 @@ static int read_ext_index_list(struct sock *sk, struct hci_dev *hdev,
 	}
 
 	rp->num_controllers = cpu_to_le16(count);
-	rp_len = sizeof(*rp) + (sizeof(rp->entry[0]) * count);
 
 	read_unlock(&hci_dev_list_lock);
 
@@ -538,7 +535,8 @@ static int read_ext_index_list(struct sock *sk, struct hci_dev *hdev,
 	hci_sock_clear_flag(sk, HCI_MGMT_UNCONF_INDEX_EVENTS);
 
 	err = mgmt_cmd_complete(sk, MGMT_INDEX_NONE,
-				MGMT_OP_READ_EXT_INDEX_LIST, 0, rp, rp_len);
+				MGMT_OP_READ_EXT_INDEX_LIST, 0, rp,
+				struct_size(rp, entry, count));
 
 	kfree(rp);