Add a Netlink spec-compatible family for netdevs.
This is a very simple implementation without much
thought going into it.

It allows us to reap all the benefits of Netlink specs,
one can use the generic client to issue the commands:

  $ ./cli.py --spec netdev.yaml --dump dev_get
  [{'ifindex': 1, 'xdp-features': set()},
   {'ifindex': 2, 'xdp-features': {'basic', 'ndo-xmit', 'redirect'}},
   {'ifindex': 3, 'xdp-features': {'rx-sg'}}]

the generic python library does not have flags-by-name
support, yet, but we also don't have to carry strings
in the messages, as user space can get the names from
the spec.
Acked-by: NJesper Dangaard Brouer <brouer@redhat.com>
Co-developed-by: NLorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: NLorenzo Bianconi <lorenzo@kernel.org>
Co-developed-by: NKumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: NKumar Kartikeya Dwivedi <memxor@gmail.com>
Co-developed-by: NMarek Majtyka <alardam@gmail.com>
Signed-off-by: NMarek Majtyka <alardam@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/327ad9c9868becbe1e601b580c962549c8cd81f2.1675245258.git.lorenzo@kernel.orgSigned-off-by: NAlexei Starovoitov <ast@kernel.org>