tools/net/ynl: Fix extack decoding for netlink-raw

Extack decoding was using a hard-coded msg header size of 20 but
netlink-raw has a header size of 16.

Use a protocol specific msghdr_size() when decoding the attr offssets.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240306231046.97158-2-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.py

@@ -353,6 +353,9 @@ class NetlinkProtocol:
             raise Exception(f'Multicast group "{mcast_name}" not present in the spec')
         return mcast_groups[mcast_name].value
 
+    def msghdr_size(self):
+        return 16
+
 
 class GenlProtocol(NetlinkProtocol):
     def __init__(self, family_name):
@@ -378,6 +381,8 @@ class GenlProtocol(NetlinkProtocol):
             raise Exception(f'Multicast group "{mcast_name}" not present in the family')
         return self.genl_family['mcast'][mcast_name]
 
+    def msghdr_size(self):
+        return super().msghdr_size() + 4
 
 
 class SpaceAttrs:
@@ -721,7 +726,7 @@ class YnlFamily(SpecFamily):
             return
 
         msg = self.nlproto.decode(self, NlMsg(request, 0, op.attr_set))
-        offset = 20 + self._struct_size(op.fixed_header)
+        offset = self.nlproto.msghdr_size() + self._struct_size(op.fixed_header)
         path = self._decode_extack_path(msg.raw_attrs, op.attr_set, offset,
                                         extack['bad-attr-offs'])
         if path: