cgroup: Drop sock_cgroup_classid() dummy implementation

The semantic of returning 0 is unclear when !CONFIG_CGROUP_NET_CLASSID.
Since there are no callers of sock_cgroup_classid() with that config
anymore we can undefine the helper at all and enforce all (future)
callers to handle cases when !CONFIG_CGROUP_NET_CLASSID.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Link: https://lore.kernel.org/r/Z_52r_v9-3JUzDT7@calendula/
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Werkov

include/linux/cgroup-defs.h

@@ -898,14 +898,12 @@ static inline u16 sock_cgroup_prioidx(const struct sock_cgroup_data *skcd)
 #endif
 }
 
+#ifdef CONFIG_CGROUP_NET_CLASSID
 static inline u32 sock_cgroup_classid(const struct sock_cgroup_data *skcd)
 {
-#ifdef CONFIG_CGROUP_NET_CLASSID
 	return READ_ONCE(skcd->classid);
-#else
-	return 0;
-#endif
 }
+#endif
 
 static inline void sock_cgroup_set_prioidx(struct sock_cgroup_data *skcd,
 					   u16 prioidx)
@@ -915,13 +913,13 @@ static inline void sock_cgroup_set_prioidx(struct sock_cgroup_data *skcd,
 #endif
 }
 
+#ifdef CONFIG_CGROUP_NET_CLASSID
 static inline void sock_cgroup_set_classid(struct sock_cgroup_data *skcd,
 					   u32 classid)
 {
-#ifdef CONFIG_CGROUP_NET_CLASSID
 	WRITE_ONCE(skcd->classid, classid);
-#endif
 }
+#endif
 
 #else	/* CONFIG_SOCK_CGROUP_DATA */