net: sk_pacing_shift_update() helper

In commit 3a9b76fd ("tcp: allow drivers to tweak TSQ logic")
I gave a code sample to set sk->sk_pacing_shift that was not complete.

Better add a helper that can be used by drivers without worries,
and maybe amended in the future.

A wifi driver might use it from its ndo_start_xmit()

Following call would setup TCP to allow up to ~8ms of queued data per
flow.

sk_pacing_shift_update(skb->sk, 7);

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