cgroup: net_cls: Move sock_update_classid() declaration to cls_cgroup.h

The only user of sock_update_classid() is net/socket.c which happens
to include cls_cgroup.h directly.

tj: Fix build breakage due to missing cls_cgroup.h inclusion in
    drivers/net/tun.c reported in linux-next by Stephen.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org
diff --git a/include/net/cls_cgroup.h b/include/net/cls_cgroup.h
index a4dc5b0..e88527a 100644
--- a/include/net/cls_cgroup.h
+++ b/include/net/cls_cgroup.h
@@ -24,6 +24,8 @@
 	u32 classid;
 };
 
+extern void sock_update_classid(struct sock *sk);
+
 #ifdef CONFIG_NET_CLS_CGROUP
 static inline u32 task_cls_classid(struct task_struct *p)
 {
@@ -62,6 +64,10 @@
 }
 #endif
 #else
+static inline void sock_update_classid(struct sock *sk)
+{
+}
+
 static inline u32 task_cls_classid(struct task_struct *p)
 {
 	return 0;