net: propagate dev_get_valid_name return code

if dev_get_valid_name failed, propagate its return code

and remove the setting err to ENODEV, it will be set to
0 again before dev_change_net_namespace exits.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>