selftests/bpf: test passing iterator to a kfunc

Define BPF iterator "getter" kfunc, which accepts iterator pointer as
one of the arguments. Make sure that argument passed doesn't have to be
the very first argument (unlike new-next-destroy combo).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240808232230.2848712-4-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>