scx_utils: clarify error about missing CONFIG_DEBUG_INFO_BTF

[arighi]

If CONFIG_DEBUG_INFO_BTF is not enabled in the kernel, the C schedulers report the following error via libbpf, clearly indicating the missing kernel config:

libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?

In contrast, the Rust schedulers report a less clear error:

thread 'main' panicked at /home/arighi/src/scx/rust/scx_utils/src/compat.rs:23:9: btf__load_vmlinux_btf() returned NULL note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

Make sure to report a similar error, so that users have a better clue about the missing kernel config. After this change the error looks like the following:

thread 'main' panicked at /home/arighi/src/scx/rust/scx_utils/src/compat.rs:23:9: btf__load_vmlinux_btf() returned NULL, was CONFIG_DEBUG_INFO_BTF enabled?

[arighi]

BTW @htejun is there any reason why we don't just depend on DEBUG_INFO_BTF on the kernel side? Like:

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index f035c87d02f1..f3d140c3acc1 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -135,7 +135,7 @@ config SCHED_CORE

 config SCHED_CLASS_EXT
    bool "Extensible Scheduling Class"
-   depends on BPF_SYSCALL && BPF_JIT
+   depends on BPF_SYSCALL && BPF_JIT && DEBUG_INFO_BTF
    help
      This option enables a new scheduler class sched_ext (SCX), which
      allows scheduling policies to be implemented as BPF programs to

[sirlucjan]

@arighi I can add it for CachyOS patchset. Just lemme know @htejun