libbpf: Improve usability of field-based CO-RE helpers

Allow to specify field reference in two ways:

  - if user has variable of necessary type, they can use variable-based
    reference (my_var.my_field or my_var_ptr->my_field). This was the
    only supported syntax up till now.
  - now, bpf_core_field_exists() and bpf_core_field_size() support also
    specifying field in a fashion similar to offsetof() macro, by
    specifying type of the containing struct/union separately and field
    name separately: bpf_core_field_exists(struct my_type, my_field).
    This forms is quite often more convenient in practice and it matches
    type-based CO-RE helpers that support specifying type by its name
    without requiring any variables.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-4-andrii@kernel.org