cls_cgroup: Fix crash on module unload

Somewhere along the lines net_cls_subsys_id became a macro when
cls_cgroup is built as a module.  Not only did it make cls_cgroup
completely useless, it also causes it to crash on module unload.

This patch fixes this by removing that macro.

Thanks to Eric Dumazet for diagnosing this problem.
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_cgroup.c

@@ -34,8 +34,6 @@ struct cgroup_subsys net_cls_subsys = {
 	.populate	= cgrp_populate,
 #ifdef CONFIG_NET_CLS_CGROUP
 	.subsys_id	= net_cls_subsys_id,
-#else
-#define net_cls_subsys_id net_cls_subsys.subsys_id
 #endif
 	.module		= THIS_MODULE,
 };