tcp: rename tcp_skb_timestamp()

This helper returns a 32bit TCP TSval from skb->tstamp.

As we are going to support usec or ms units soon, rename it
to tcp_skb_timestamp_ts() and add a boolean to select the unit.

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