Wrap `do_fast_fallback_getaddrinfo` with `rb_thread_prevent_fork`

Referencing PR #10864,
wrap `do_fast_fallback_getaddrinfo` with `rb_thread_prevent_fork`
to avoid fork safety issues.

`do_fast_fallback_getaddrinfo` internally uses getaddrinfo(3),
leading to fork safety issues, as described in PR #10864.
This change ensures that `do_fast_fallback_getaddrinfo`
is guarded by `rb_thread_prevent_fork`,
preventing fork during its execution and avoiding related issues.