[NET]: Cleanup pernet operation without CONFIG_NET_NS

If CONFIG_NET_NS is not set, the only namespace is possible.

This patch removes list of pernet_operations and cleanups code a bit.
This list is not needed if there are no namespaces. We should just call
->init method.

Additionally, the ->exit will be called on module unloading only. This
case is safe - the code is not discarded. For the in/kernel code, ->exit
should never be called.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>