bpf: allow passing struct bpf_iter_<type> as kfunc arguments

bpf: allow passing struct bpf_iter_<type> as kfunc arguments

There are potentially useful cases where a specific iterator type might
need to be passed into some kfunc. So, in addition to existing
bpf_iter_<type>_{new,next,destroy}() kfuncs, allow to pass iterator
pointer to any kfunc.

We employ "__iter" naming suffix for arguments that are meant to accept
iterators. We also enforce that they accept PTR -> STRUCT btf_iter_<type>
type chain and point to a valid initialized on-the-stack iterator state.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240808232230.2848712-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>