[XFRM]: Fix crash introduced by struct dst_entry reordering

XFRM expects xfrm_dst->u.next to be same pointer as dst->next, which
was broken by the dst_entry reordering in commit 1e19e02c~, causing
an oops in xfrm_bundle_ok when walking the bundle upwards.

Kill xfrm_dst->u.next and change the only user to use dst->next instead.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 157bfbd..b48f06f 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2141,7 +2141,7 @@
 		if (last == first)
 			break;
 
-		last = last->u.next;
+		last = (struct xfrm_dst *)last->u.dst.next;
 		last->child_mtu_cached = mtu;
 	}