tools/net/ynl: Fix extack parsing with fixed header genlmsg

Move decode_fixed_header into YnlFamily and add a _fixed_header_size
method to allow extack decoding to skip the fixed header.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-7-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>