Fix incorrect pinned maps handling and missing userspace setup code

[SiyuanSun0736]

When using the pin keyword with map declarations (e.g., pin var cpu_counters : array<...>), the current implementation faces two primary issues that prevent the maps from being correctly pinned to the BPF filesystem.

The Problems

1. Incorrect Treatment as Global Variables: The IR generator incorrectly creates both an IR map definition and an IR global variable for pinned maps. This causes the userspace codegen to treat the map as a "pinned global variable," looking for a non-existent __pinned_globals map and resulting in the error: Failed to find pinned globals map in eBPF object.
2. Missing Setup Code: Setup and pinning logic for maps is only generated if the userspace code explicitly performs map operations. However, pinned maps require setup (pinning to /sys/fs/bpf/...) even if they are only accessed within the eBPF program itself.
3. Variable Redefinition: If multiple pinned maps exist, the generated C code uses the same variable name (existing_fd) for all of them, leading to compilation errors.
4. Missing Directory Creation: The generated code fails to pin maps if the parent directory (e.g., /sys/fs/bpf/<project>/maps/) does not already exist.

Proposed Changes

The provided patch addresses these issues with the following logic:

• IR Generator: Modified ir_generator.ml to ensure that map-type global declarations only produce map definitions, not global variable entries.
• Userspace Codegen:
• Updated userspace_codegen.ml to trigger map setup and FD declarations if any pinned maps are detected (has_pinned_maps), regardless of whether they are used in userspace C code.
• Implemented unique variable naming (e.g., %s_existing_fd) to prevent C redefinition errors.
• Added a C helper function ensure_bpf_dir to recursively create the necessary BPF filesystem directories before pinning.
• Included necessary headers (<sys/stat.h>, <sys/types.h>) when pinning is required.

fix_pin_var_map.patch

Verification

Testing with examples/maps_demo.ks confirms:

• The "Failed to find pinned globals map" error is resolved.
• Maps are correctly pinned to /sys/fs/bpf/maps_demo/maps/.
• Userspace programs compile and run correctly with multiple pinned maps.