[IPSEC]: Fix xfrm[46]_update_pmtu to update top dst

Let's also fix IPsec PMTU storage.  When we get an MTU update for an
xfrm_dst, it should be done to the top dst, not the bottom dst.

For example, when we get a need-to-frag message for host C behind
a our IPsec peer B, we should be updating the dst entry for C and
not B as we do now.

I've removed the boundary checks since the same checks are done
in ipv[46]/route.c already.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_policy.c

@@ -235,10 +235,8 @@ static inline int xfrm4_garbage_collect(void)
 
 static void xfrm4_update_pmtu(struct dst_entry *dst, u32 mtu)
 {
-	struct dst_entry *path = dst->path;
-
-	if (mtu < 68 + dst->header_len)
-		return;
+	struct xfrm_dst *xdst = (struct xfrm_dst *)dst;
+	struct dst_entry *path = xdst->route;
 
 	path->ops->update_pmtu(path, mtu);
 }

net/ipv6/xfrm6_policy.c

@@ -243,12 +243,10 @@ static inline int xfrm6_garbage_collect(void)
 
 static void xfrm6_update_pmtu(struct dst_entry *dst, u32 mtu)
 {
-	struct dst_entry *path = dst->path;
+	struct xfrm_dst *xdst = (struct xfrm_dst *)dst;
+	struct dst_entry *path = xdst->route;
 
-	if (mtu >= IPV6_MIN_MTU && mtu < dst_pmtu(dst))
 	path->ops->update_pmtu(path, mtu);
-	
-	return;
 }
 
 static struct dst_ops xfrm6_dst_ops = {