vlan: fail early when creating netdev named config

Similarly, vlan will create  /proc/net/vlan/<dev>, so when we
create dev with name "config", it will confict with
/proc/net/vlan/config.

Reported-by: Stephane Chazelas <stephane.chazelas@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>