bpf: add btf pointer to struct bpf_ctx_arg_aux.

Enable the providers to use types defined in a module instead of in the
kernel (btf_vmlinux).

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240209023750.1153905-2-thinker.li@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>