net: #ifdefify sk_classid member of struct sock

The sk_classid member is only required when CONFIG_CGROUP_NET_CLASSID is
enabled. #ifdefify it to reduce the size of struct sock on 32 bit
systems, at least.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock.h b/include/net/sock.h
index 05a8c1a..4353ef7 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -429,7 +429,9 @@
 	void			*sk_security;
 #endif
 	__u32			sk_mark;
+#ifdef CONFIG_CGROUP_NET_CLASSID
 	u32			sk_classid;
+#endif
 	struct cg_proto		*sk_cgrp;
 	void			(*sk_state_change)(struct sock *sk);
 	void			(*sk_data_ready)(struct sock *sk);