net: loopback: clear skb->tstamp before netif_rx()

At least UDP / TCP stacks can now cook skbs with a tstamp using
MONOTONIC base (or arbitrary values with SCM_TXTIME)

Since loopback driver does not call (directly or indirectly)
skb_scrub_packet(), we need to clear skb->tstamp so that
net_timestamp_check() can eventually resample the time,
using ktime_get_real().

Fixes: 80b14de ("net: Add a new socket option for a future transmit time.")
Fixes: fb420d5 ("tcp/fq: move back to CLOCK_MONOTONIC")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -69,6 +69,10 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
 	int len;
 
 	skb_tx_timestamp(skb);
+
+	/* do not fool net_timestamp_check() with various clock bases */
+	skb->tstamp = 0;
+
 	skb_orphan(skb);
 
 	/* Before queueing this packet to netif_rx(),