Comment 12 for bug 1763454

(I used my team account cloudstackers-7 before, now with my own one)

The bounds check may not be necessary, because replace_map_fd_with_map_ptr is called before do_check and the relevant check is already in replace_map_fd_with_map_ptr. But it's not obvious, so at least a comment in do_check may be a good idea.