user_ns: get rid of duplicate code in net_ctl_permissions

Get rid of duplicate code in net_ctl_permissions and fix the comment.

Signed-off-by: Zhao Hongjiang <zhaohongjiang@huawei.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>