rtnetlink: Clean up rtnl_dellink().

We will push RTNL down to rtnl_delink().

Let's unify the error path to make it easy to place rtnl_net_lock().

While at it, keep the variables in reverse xmas order.

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