tcp: replace tcp_time_stamp_raw()

In preparation of usec TCP TS support, remove tcp_time_stamp_raw()
in favor of tcp_clock_ts() helper. This helper will return a suitable
32bit result to feed TS values, depending on a socket field.

Also add tcp_tw_tsval() and tcp_rsk_tsval() helpers to factorize
the details.

We do not yet support usec timestamps.

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