[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:
1567ca7e
3edf8fa5
2d38f9a4

Revert them.

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