selftests/bpf: check operations on untrusted ro pointers to mem

selftests/bpf: check operations on untrusted ro pointers to mem

The following cases are tested:
- it is ok to load memory at any offset from rdonly_untrusted_mem;
- rdonly_untrusted_mem offset/bounds are not tracked;
- writes into rdonly_untrusted_mem are forbidden;
- atomic operations on rdonly_untrusted_mem are forbidden;
- rdonly_untrusted_mem can't be passed as a memory argument of a
  helper of kfunc;
- it is ok to use PTR_TO_MEM and PTR_TO_BTF_ID in a same load
  instruction.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250625182414.30659-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>