net: create netdev_nl_sock to wrap bindings list

No functional changes. Next patches will add more granular locking
to netdev_nl_sock.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250311144026.4154277-2-sdf@fomichev.me


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