netns: exclude ipvs from struct net when IPVS disabled

no real problem is fixed, just save a few bytes in
net_namespace structure.
Signed-off-by: JunweiZhang <junwei.zhang@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

include/net/net_namespace.h

@@ -115,7 +115,9 @@ struct net {
 #ifdef CONFIG_XFRM
 	struct netns_xfrm	xfrm;
 #endif
+#if IS_ENABLED(CONFIG_IP_VS)
 	struct netns_ipvs	*ipvs;
+#endif
 	struct sock		*diag_nlsk;
 	atomic_t		rt_genid;
 	atomic_t		fnhe_genid;