netlink: specs: support nested structs in genetlink legacy

Nested structs are already supported in netlink-raw. Add the same
capability to the genetlink legacy schema.

Signed-off-by: Donald Hunter <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: donaldh

Documentation/netlink/genetlink-legacy.yaml

@@ -152,6 +152,9 @@ properties:
                   the right formatting mechanism when displaying values of this
                   type.
                 enum: [ hex, mac, fddi, ipv4, ipv6, uuid ]
+              struct:
+                description: Name of the nested struct type.
+                type: string
         # End genetlink-legacy
 
   attribute-sets: