Change to kcompat system

[amd-vserbu]

Summary

Replace the LINUX_VERSION_CODE gates in the SLASH kernel module with a small build-time feature-probe system. The driver Makefile now runs driver/kcompat/probe.sh against $(KDIR), which compiles one conftest .c per feature against the target kernel and prints SLASH_HAVE_<FEAT>=y|n. The results are passed back into the kbuild recursion as both make variables and -DSLASH_HAVE_<FEAT> ccflags, and driver/slash_compat.h selects the modern or legacy API form based on those defines.

Why

This replaces the existing LINUX_VERSION_CODE >= KERNEL_VERSION(...) checks in the driver, which is brittle on distro kernels that backport features from newer upstreams (e.g. RHEL 9.x) and breaks the moment a new LTS picks up another behavior change. Conftest probes give us the actual ground truth for the kernel we are building against.

Feature flags

Two features are probed today:

• SLASH_HAVE_VM_FLAGS_SET -- vm_flags_set() was added in 6.3. Old kernels fall back to vma->vm_flags |= flags. New kernels cannot use the old form because vma->vm_flags is now const/read-only.
• SLASH_HAVE_MODULE_IMPORT_NS_TOKEN -- MODULE_IMPORT_NS() switched from bare-token to string-literal in 6.13. The probe checks the token form directly, so the result is precise across the cutover (the string form silently produces the wrong namespace on older kernels and is not safe to probe).

[hpc-aulmamei]

lgtm

[quetric]

@amd-vserbu could you please make sure all files have license headers?

[amd-vserbu]

I've added the missing license headers.