tcp: introduce tcp_clock_ms()

It delivers current TCP time stamp in ms unit, and is used
in place of confusing tcp_time_stamp_raw()

It is the same family than tcp_clock_ns() and tcp_clock_ms().

tcp_time_stamp_raw() will be replaced later for TSval
contexts with a more descriptive name.

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