xfrm: Make function xfrmi_get_link_net() static

Fixes the following sparse warning:

net/xfrm/xfrm_interface.c:745:12: warning:
 symbol 'xfrmi_get_link_net' was not declared. Should it be static?

Fixes: f203b76d ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_interface.c

@@ -742,7 +742,7 @@ static int xfrmi_fill_info(struct sk_buff *skb, const struct net_device *dev)
 	return -EMSGSIZE;
 }
 
-struct net *xfrmi_get_link_net(const struct net_device *dev)
+static struct net *xfrmi_get_link_net(const struct net_device *dev)
 {
 	struct xfrm_if *xi = netdev_priv(dev);