tun: use sk_fullsock() before reading sk->sk_tsflags

timewait or request sockets are small and do not contain sk->sk_tsflags

Without this fix, we might read garbage, and crash later in

__skb_complete_tx_timestamp()
 -> sock_queue_err_skb()

(These pseudo sockets do not have an error queue either)

Fixes: ca6fb0651883 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -858,7 +858,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
 		goto drop;
 
-	if (skb->sk) {
+	if (skb->sk && sk_fullsock(skb->sk)) {
 		sock_tx_timestamp(skb->sk, &skb_shinfo(skb)->tx_flags);
 		sw_tx_timestamp(skb);
 	}