bpf: rdonly_untrusted_mem for btf id walk pointer leafs

bpf: rdonly_untrusted_mem for btf id walk pointer leafs

When processing a load from a PTR_TO_BTF_ID, the verifier calculates
the type of the loaded structure field based on the load offset.
For example, given the following types:

  struct foo {
    struct foo *a;
    int *b;
  } *p;

The verifier would calculate the type of `p->a` as a pointer to
`struct foo`. However, the type of `p->b` is currently calculated as a
SCALAR_VALUE.

This commit updates the logic for processing PTR_TO_BTF_ID to instead
calculate the type of p->b as PTR_TO_MEM|MEM_RDONLY|PTR_UNTRUSTED.
This change allows further dereferencing of such pointers (using probe
memory instructions).

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250704230354.1323244-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>