[NETNS]: Fix allnoconfig compilation error.

When CONFIG_NET=no, init_net is unresolved because net_namespace.c
is not compiled and the include pull init_net definition.

This problem was very similar with the ipc namespace where the kernel
can be compiled with SYSV ipc out.

This patch fix that defining a macro which simply remove init_net
initialization from nsproxy namespace aggregator.

Compiled and booted on qemu-i386 with CONFIG_NET=no and CONFIG_NET=yes.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index fac42db..3081b6e 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -28,7 +28,14 @@
 	struct hlist_head	*dev_index_head;
 };
 
+#ifdef CONFIG_NET
+/* Init's network namespace */
 extern struct net init_net;
+#define INIT_NET_NS(net_ns) .net_ns = &init_net,
+#else
+#define INIT_NET_NS(net_ns)
+#endif
+
 extern struct list_head net_namespace_list;
 
 extern void __put_net(struct net *net);