bpf: Prepare to reuse get_ctx_arg_idx

Rename get_ctx_arg_idx to bpf_ctx_arg_idx, and allow others to call it.
No functional change.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20250409214606.2000194-2-ameryhung@gmail.com