net sysctl: Add place holder functions for when sysctl support is compiled out of the kernel.

Randy Dunlap <rdunlap@xenotime.net> reported:
> On 04/23/2012 12:07 AM, Stephen Rothwell wrote:
>
>> Hi all,
>>
>> Changes since 20120420:
>
>
>
> ERROR: "unregister_net_sysctl_table" [net/phonet/phonet.ko] undefined!
> ERROR: "register_net_sysctl" [net/phonet/phonet.ko] undefined!
>
> when CONFIG_SYSCTL is not enabled.

Add static inline stub functions to gracefully handle the case when sysctl
support is not present.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 3ee4a3d..ac9195e 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -284,11 +284,20 @@
 
 #ifdef CONFIG_SYSCTL
 extern int net_sysctl_init(void);
-#else
-static inline int net_sysctl_init(void) { return 0; }
-#endif
 extern struct ctl_table_header *register_net_sysctl(struct net *net,
 	const char *path, struct ctl_table *table);
 extern void unregister_net_sysctl_table(struct ctl_table_header *header);
+#else
+static inline int net_sysctl_init(void) { return 0; }
+static inline struct ctl_table_header *register_net_sysctl(struct net *net,
+	const char *path, struct ctl_table *table)
+{
+	return NULL;
+}
+static inline void unregister_net_sysctl_table(struct ctl_table_header *header)
+{
+}
+#endif
+
 
 #endif /* __NET_NET_NAMESPACE_H */