Fix syscall_info

[cbarrete]

What does this PR do

It fixes ptrace::syscall_info, which is currently broken (at least on Linux 6.12.31).

See the commit message for more details.

Reproduction

The following program panics when built against master and behaves correctly (i.e. it does not panic and prints valid syscall info) when built against this branch:

fn main() {
    let pid = unsafe { nix::unistd::fork().unwrap() };

    match pid {
        nix::unistd::ForkResult::Child => {
            nix::sys::ptrace::traceme().unwrap();
            nix::sys::signal::raise(nix::sys::signal::Signal::SIGCONT).unwrap();
            println!("I'm issuing a syscall!");
        }
        nix::unistd::ForkResult::Parent { child } => {
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            nix::sys::ptrace::setoptions(child, nix::sys::ptrace::Options::PTRACE_O_TRACESYSGOOD)
                .unwrap();

            nix::sys::ptrace::syscall(child, None).unwrap();
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
            println!("{syscall_info:?}");
            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_ENTRY);

            nix::sys::ptrace::syscall(child, None).unwrap();
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
            println!("{syscall_info:?}");
            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_EXIT);
        }
    }
}

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API