libbpf: __arg_untrusted in bpf_helpers.h

libbpf: __arg_untrusted in bpf_helpers.h

Make btf_decl_tag("arg:untrusted") available for libbpf users via
macro. Makes the following usage possible:

  void foo(struct bar *p __arg_untrusted) { ... }
  void bar(struct foo *p __arg_trusted) {
    ...
    foo(p->buz->bar); // buz derefrence looses __trusted
    ...
  }

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250704230354.1323244-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>