net-tcp: fix panic in tcp_fastopen_cache_set()

We had some reports of crashes using TCP fastopen, and Dave Jones
gave a nice stack trace pointing to the error.

Issue is that tcp_get_metrics() should not be called with a NULL dst

Fixes: 1fe4c48 ("net-tcp: Fast Open client - cookie cache")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dave Jones <davej@redhat.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Tested-by: Dave Jones <davej@fedoraproject.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_metrics.c

@@ -663,10 +663,13 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
 void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
 			    struct tcp_fastopen_cookie *cookie, bool syn_lost)
 {
+	struct dst_entry *dst = __sk_dst_get(sk);
 	struct tcp_metrics_block *tm;
 
+	if (!dst)
+		return;
 	rcu_read_lock();
-	tm = tcp_get_metrics(sk, __sk_dst_get(sk), true);
+	tm = tcp_get_metrics(sk, dst, true);
 	if (tm) {
 		struct tcp_fastopen_metrics *tfom = &tm->tcpm_fastopen;