Add variant of close_all_fds() that does not allocate and use it in freeze()

Even though it's just a fallback path, let's not be sloppy and allocate in
the crash handler.

> The deadlock happens because systemd crash in malloc() then in signal
> handler, it calls malloc() (close_all_fds()-> opendir()-> __alloc_dir())
> again. malloc() is not a signal-safe function, maybe we should re-think
> the logic here.

Fixes #20266.