[NETNS]: Declare init_net even without CONFIG_NET defined.

This does not look good, but there is no other choice. The compilation
without CONFIG_NET is broken and can not be fixed with ease.

After that there is no need for the following commits:
1567ca7eec7664b8be3b07755ac59dc1b1ec76cb
3edf8fa5ccf10688a9280b5cbca8ed3947c42866
2d38f9a4f8d2ebdc799f03eecf82345825495711

Revert them.

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