[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: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -585,7 +585,6 @@ static inline int xfrm_sec_ctx_match(struct xfrm_sec_ctx *s1, struct xfrm_sec_ct
 struct xfrm_dst
 {
 	union {
-		struct xfrm_dst		*next;
 		struct dst_entry	dst;
 		struct rtable		rt;
 		struct rt6_info		rt6;

net/xfrm/xfrm_policy.c

@@ -2141,7 +2141,7 @@ int xfrm_bundle_ok(struct xfrm_policy *pol, struct xfrm_dst *first,
 		if (last == first)
 			break;
 
-		last = last->u.next;
+		last = (struct xfrm_dst *)last->u.dst.next;
 		last->child_mtu_cached = mtu;
 	}