Bluetooth: hci_conn: Use struct_size() in hci_le_big_create_sync()

Use struct_size() instead of the open-coded version. Similarly to
this other patch[1].

Link: https://lore.kernel.org/linux-hardening/ZiwwPmCvU25YzWek@neat/ [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/hci_conn.c

@@ -2132,7 +2132,7 @@ int hci_le_big_create_sync(struct hci_dev *hdev, struct hci_conn *hcon,
 	memcpy(pdu->bis, bis, num_bis);
 
 	return hci_send_cmd(hdev, HCI_OP_LE_BIG_CREATE_SYNC,
-			    sizeof(*pdu) + num_bis, pdu);
+			    struct_size(pdu, bis, num_bis), pdu);
 }
 
 static void create_big_complete(struct hci_dev *hdev, void *data, int err)