tcp: introduce TCP_PAWS_WRAP

tcp_paws_check() uses TCP_PAWS_24DAYS constant to detect if TCP TS
values might have wrapped after a long idle period.

This mechanism is described in RFC 7323 5.5 (Outdated Timestamps)

TCP_PAWS_24DAYS value was based on the assumption of a clock
of 1 Khz.

As we want to adopt a 1 Mhz clock in the future, we reduce
this constant.

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