tcp: switch tcp_clock_ns() to CLOCK_TAI base

TCP pacing is either implemented in sch_fq or internally.
We have the goal of being able to offload pacing on the NICS.

TCP will soon provide per skb skb->tstamp as early departure time.

Like ETF in commit 25db26a9

 ("net/sched: Introduce the ETF Qdisc")
we chose CLOCK_T as the clock base, so that TCP and pacers can share
a common clock, to get better RTT samples (without pacing artificially
inflating these samples).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>