net: Clear mono_delivery_time bit in __skb_tstamp_tx()

In __skb_tstamp_tx(), it may clone the egress skb and queues the clone to
the sk_error_queue.  The outgoing skb may have the mono delivery_time
while the (rcv) timestamp is expected for the clone, so the
skb->mono_delivery_time bit needs to be cleared from the clone.

This patch adds the skb->mono_delivery_time clearing to the existing
__net_timestamp() and use it in __skb_tstamp_tx().
The __net_timestamp() fast path usage in dev.c is changed to directly
call ktime_get_real() since the mono_delivery_time bit is not set at
that point.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3981,6 +3981,7 @@ static inline void skb_get_new_timestampns(const struct sk_buff *skb,
 static inline void __net_timestamp(struct sk_buff *skb)
 {
 	skb->tstamp = ktime_get_real();
+	skb->mono_delivery_time = 0;
 }
 
 static inline ktime_t net_timedelta(ktime_t t)

net/core/dev.c

@@ -2111,13 +2111,13 @@ static inline void net_timestamp_set(struct sk_buff *skb)
 	skb->tstamp = 0;
 	skb->mono_delivery_time = 0;
 	if (static_branch_unlikely(&netstamp_needed_key))
-		__net_timestamp(skb);
+		skb->tstamp = ktime_get_real();
 }
 
 #define net_timestamp_check(COND, SKB)				\
 	if (static_branch_unlikely(&netstamp_needed_key)) {	\
 		if ((COND) && !(SKB)->tstamp)			\
-			__net_timestamp(SKB);			\
+			(SKB)->tstamp = ktime_get_real();	\
 	}							\
 
 bool is_skb_forwardable(const struct net_device *dev, const struct sk_buff *skb)

net/core/skbuff.c

@@ -4851,7 +4851,7 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
 	if (hwtstamps)
 		*skb_hwtstamps(skb) = *hwtstamps;
 	else
-		skb->tstamp = ktime_get_real();
+		__net_timestamp(skb);
 
 	__skb_complete_tx_timestamp(skb, sk, tstype, opt_stats);
 }