tcp: Remove one extra ktime_get_ns() from cookie_init_timestamp

tcp_make_synack() already uses tcp_clock_ns(), and can pass
the value to cookie_init_timestamp() to avoid another call
to ktime_get_ns() helper.

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