[NETNS]: Compile NET /proc support only if CONFIG_NET is set.

This fix broken compilation for 'allnoconfig'. This was introduced by
Introduced by commit 1218854a ("[NET]
NETNS: Omit seq_net_private->net without CONFIG_NET_NS.")
Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/proc/proc_net.c

@@ -51,6 +51,7 @@ int seq_open_net(struct inode *ino, struct file *f,
 }
 EXPORT_SYMBOL_GPL(seq_open_net);
 
+#ifdef CONFIG_NET
 int seq_release_net(struct inode *ino, struct file *f)
 {
 	struct seq_file *seq;
@@ -218,3 +219,4 @@ int __init proc_net_init(void)
 
 	return register_pernet_subsys(&proc_net_ns_ops);
 }
+#endif /* CONFIG_NET */

include/linux/seq_file.h

@@ -63,6 +63,7 @@ extern struct list_head *seq_list_start_head(struct list_head *head,
 extern struct list_head *seq_list_next(void *v, struct list_head *head,
 		loff_t *ppos);
 
+#ifdef CONFIG_NET
 struct net;
 struct seq_net_private {
 #ifdef CONFIG_NET_NS
@@ -81,6 +82,7 @@ static inline struct net *seq_file_net(struct seq_file *seq)
 	return &init_net;
 #endif
 }
+#endif	/* CONFIG_NET */
 
 #endif
 #endif