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 <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250211120127.84858-9-donald.hunter@gmail.com


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