Collect MTU from skb->_skb_refdst

According to source code, kernel uses MTU from skb->_skb_refdst. Let
pwru collect MTU from there using the same logic.

It requires to cast skb->_skb_refdst to dst_entry*, then fetch
dst_metric_raw(dst, RTAX_MTU) and dst->dev->mtu.

```
// https://elixir.bootlin.com/linux/v6.5/source/net/ipv4/ip_forward.c#L86
// net/ipv4/ip_forward.c
int ip_forward(struct sk_buff *skb)
{
[...]
	rt = skb_rtable(skb);
[...]
	mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
	if (ip_exceeds_mtu(skb, mtu)) {
		IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
			  htonl(mtu));
		SKB_DR_SET(reason, PKT_TOO_BIG);
		goto drop;
	}
[...]
}

// include/linux/skbuff.h
static inline struct rtable *skb_rtable(const struct sk_buff *skb)
{
	return (struct rtable *)skb_dst(skb);
}

// include/linux/skbuff.h
static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
{
[...]
	return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
}

// include/net/ip.h
static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
						    bool forwarding)
{
[...]
	mtu = dst_metric_raw(dst, RTAX_MTU);
	if (mtu)
		goto out;

	mtu = READ_ONCE(dst->dev->mtu);
[...]
}

// include/net/dst.h
	((u32 *)((Y) & ~DST_METRICS_FLAGS))
```

With this patch, pwru can output the correct MTU used by OS.

Case 1: Cilium could reduce the route MTU to 1423 inside a pod. This
can't be detected by pwru because it only checks link MTU.

Case 2: Xfrm could reduce the route MTU to 1446 in ip_forward(). Pwru
must inspect route MTU from skb->_skb_dstref to understand that.

Signed-off-by: gray <[email protected]>

bpf/kprobe_pwru.c

@@ -14,6 +14,17 @@
 #define ETH_P_IP              0x800
 #define ETH_P_IPV6            0x86dd
 
+#define RTAX_MTU              2
+#define SKB_DST_NOREF         1UL
+#define SKB_DST_PTRMASK       ~(SKB_DST_NOREF)
+#define __SKB_DST_PTR(X)      \
+	((struct dst_entry *)((X) & SKB_DST_PTRMASK))
+
+#define DST_METRICS_FLAGS     0x3UL
+#define __DST_METRICS_PTR(X)  \
+	((u32 *)((X) & ~DST_METRICS_FLAGS))
+
+
 const static bool TRUE = true;
 
 volatile const static __u64 BPF_PROG_ADDR = 0;
@@ -230,6 +241,13 @@ set_meta(struct sk_buff *skb, struct skb_meta *meta) {
 	meta->protocol = BPF_CORE_READ(skb, protocol);
 	meta->ifindex = BPF_CORE_READ(skb, dev, ifindex);
 	meta->mtu = BPF_CORE_READ(skb, dev, mtu);
+	struct dst_entry *dst = __SKB_DST_PTR(BPF_CORE_READ(skb, _skb_refdst));
+	if (dst) {
+		u32 *metrics = __DST_METRICS_PTR(BPF_CORE_READ(dst, _metrics));
+		bpf_probe_read_kernel(&meta->mtu, sizeof(meta->mtu), metrics + RTAX_MTU - 1);
+		if (!meta->mtu)
+			meta->mtu = BPF_CORE_READ(dst, dev, mtu);
+	}
 }
 
 static __always_inline void