rtnetlink: Add ASSERT_RTNL_NET() placeholder for netdev notifier.

The global and per-netns netdev notifier depend on RTNL, and its
dependency is not so clear due to nested calls.

Let's add a placeholder to place ASSERT_RTNL_NET() for each event.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>