bpf: generalize dynptr_get_spi to be usable for iters

Generalize the logic of fetching special stack slot object state using
spi (stack slot index). This will be used by STACK_ITER logic next.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-12-andrii@kernel.org


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