[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: NDaniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/init_task.h

@@ -79,7 +79,7 @@ extern struct nsproxy init_nsproxy;
 	.nslock		= __SPIN_LOCK_UNLOCKED(nsproxy.nslock),		\
 	.uts_ns		= &init_uts_ns,					\
 	.mnt_ns		= NULL,						\
-	.net_ns		= &init_net,					\
+	INIT_NET_NS(net_ns)                                             \
 	INIT_IPC_NS(ipc_ns)						\
 	.user_ns	= &init_user_ns,				\
 }

include/net/net_namespace.h

@@ -28,7 +28,14 @@ struct net {
 	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);