tools/net/ynl: Combine struct decoding logic in ynl

_decode_fixed_header() and NlAttr.as_struct() both implemented struct
decoding logic. Deduplicate the code into newly named _decode_struct()
method.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-8-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>