conncheck: Don't remove elements in the conncheck list while iterating it

conncheck: Don't remove elements in the conncheck list while iterating it

priv_limit_conn_check_list_size() would remove elemtns from the conncheck_list
while the calling functions were iterating it. Now instead just mark them as
cancelled. Then later, at the outer function, free all cancelled elements to
prevent the list from growing out of bounds.