fix: use 1ULL instead of 1 in CPU bitmap shift to avoid UB

[Inquisitor-201]

Summary

• Fix undefined behavior in CPU bitmap construction when cpu_id >= 31
• 1 << cpu_id uses a signed int — 1 << 31 is UB (sign-bit overflow),
  and 1 << 32 is UB on 64-bit (shift >= width of type)
• Change to 1ULL << cpu_id for a well-defined 64-bit shift
• Current max platform is 16 CPUs, but this is a latent bug waiting to trigger
  on any platform with 31+ CPUs or with a misconfigured JSON config

[agicy]

Why not use a bitmap? Using 1ull still gets wrong when cpu_id >= 64.

[Inquisitor-201]

@agicy Good point, but the cpus field is __u64 on both sides, so the interface already caps at 64 CPUs. Switching to a bitmap would require changing struct layouts and the hypercall ABI across both repos — too risky for no real benefit since current platforms max out at 16 CPUs. Happy to revisit if >64 CPU support is needed later.