bpf: Factor btf_struct_access function

Adding btf_struct_walk function that walks through the
struct type + given offset and returns following values:

  enum bpf_struct_walk_result {
       /* < 0 error */
       WALK_SCALAR = 0,
       WALK_PTR,
       WALK_STRUCT,
  };

WALK_SCALAR - when SCALAR_VALUE is found
WALK_PTR    - when pointer value is found, its ID is stored
              in 'next_btf_id' output param
WALK_STRUCT - when nested struct object is found, its ID is stored
              in 'next_btf_id' output param

It will be used in following patches to get all nested
struct objects for given type and offset.

The btf_struct_access now calls btf_struct_walk function,
as long as it gets nested structs as return value.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-8-jolsa@kernel.org