bpf: Zero index arg error string for dynptr and iter

Andrii spotted that process_dynptr_func's rejection of incorrect
argument register type will print an error string where argument numbers
are not zero-indexed, unlike elsewhere in the verifier.  Fix this by
subtracting 1 from regno. The same scenario exists for iterator
messages. Fix selftest error strings that match on the exact argument
number while we're at it to ensure clean bisection.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241203002235.3776418-1-memxor@gmail.com


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