[ynl] how to fix the order of selector and `_decode_sub_msg`

[Hangbin Liu <hangbin.liu@linux.dev>]

Hi Donald, Jakub,

Recently I have been trying to add encap decoding support for `rt‑route.yaml`,
with formats similar to:

      -
        name: encap-type
        type: u16
        enum: encap-type
      -
        name: encap
        type: sub-message
        sub-message: encap-data
        selector: encap-type

However, sub‑message decoding is broken inside `ynl.py`. The root cause is
that `lwtunnel_fill_encap()` sets the encap‑type *after* the encap payload
data.

In iproute2, `parse_rtattr()` extracts all attributes together. In contrast,
`ynl.py` stores attributes one‑by‑one into `search_attrs` after they get
decoded (_decode() -> self._rsp_add()). As a result, we cannot locate the
`selector` while decoding the "encap" payload.

Should we decode sub‑messages after finishing first‑level attribute decoding?
Do you have any suggestions on how we can fix this?

Thanks
Hangbin