bpf: extract iterator argument type and name validation logic

Verifier enforces that all iterator structs are named `bpf_iter_<name>`
and that whenever iterator is passed to a kfunc it's passed as a valid PTR ->
STRUCT chain (with potentially const modifiers in between).

We'll need this check for upcoming changes, so instead of duplicating
the logic, extract it into a helper function.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240808232230.2848712-2-andrii@kernel.org


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