fix for OOPS in pernet list operations if CONFIG_NET_NS undefined

These patches are addressed to the oops reported by the Cedric Le Goater a week ago. The pernet_operations were discarder during kernel boot and this breaks further operations as this

Though, the patch from Pavel Emelyanov was partially reverted by the Eric W. Biederman [commit 2b008b0a8e96b726c603c5e1a5a7a509b5f61e35]

So, I revert the Eric patch (actually, Eric one can be simply dropped) and fix original code. There is no need for such complex code if CONFIG_NET_NS is not defined.