memcg, kmem: fix build error when CONFIG_INET is disabled

Commit e1aab161 ("socket: initial cgroup code.") causes a build
error when CONFIG_INET is disabled in Linus' tree:

  net/built-in.o: In function `sk_update_clone':
  net/core/sock.c:1336: undefined reference to `sock_update_memcg'

sock_update_memcg() is only defined when CONFIG_INET is enabled, so fix
it by defining the dummy function without this option.
Signed-off-by: David Rientjes <rientjes@google.com>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -396,7 +396,7 @@ enum {
 };
 
 struct sock;
-#ifdef CONFIG_MEMCG_KMEM
+#if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
 void sock_update_memcg(struct sock *sk);
 void sock_release_memcg(struct sock *sk);
 #else
@@ -406,6 +406,6 @@ static inline void sock_update_memcg(struct sock *sk)
 static inline void sock_release_memcg(struct sock *sk)
 {
 }
-#endif /* CONFIG_MEMCG_KMEM */
+#endif /* CONFIG_INET && CONFIG_MEMCG_KMEM */
 #endif /* _LINUX_MEMCONTROL_H */