tcp: provide earliest departure time in skb->tstamp

Switch internal TCP skb->skb_mstamp to skb->skb_mstamp_ns,
from usec units to nsec units.

Do not clear skb->tstamp before entering IP stacks in TX,
so that qdisc or devices can implement pacing based on the
earliest departure time instead of socket sk->sk_pacing_rate

Packets are fed with tcp_wstamp_ns, and following patch
will update tcp_wstamp_ns when both TCP and sch_fq switch to
the earliest departure time mechanism.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index e3a53ca..86f337e 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -689,7 +689,7 @@ struct sk_buff {
 
 	union {
 		ktime_t		tstamp;
-		u64		skb_mstamp;
+		u64		skb_mstamp_ns; /* earliest departure time */
 	};
 	/*
 	 * This is the control buffer. It is free to use for every