net: Create and use new helper xfrm_dst_child().

Only IPSEC routes have a non-NULL dst->child pointer.  And IPSEC
routes are identified by a non-NULL dst->xfrm pointer.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/security/selinux/xfrm.c b/security/selinux/xfrm.c
index 56e354f..92818890 100644
--- a/security/selinux/xfrm.c
+++ b/security/selinux/xfrm.c
@@ -452,7 +452,7 @@ int selinux_xfrm_postroute_last(u32 sk_sid, struct sk_buff *skb,
 	if (dst) {
 		struct dst_entry *iter;
 
-		for (iter = dst; iter != NULL; iter = iter->child) {
+		for (iter = dst; iter != NULL; iter = xfrm_dst_child(iter)) {
 			struct xfrm_state *x = iter->xfrm;
 
 			if (x && selinux_authorizable_xfrm(x))