Don't expose the uptime via the TCP timestamps.

The TCP client side or the TCP server side when not using SYN-cookies
used the uptime as the TCP timestamp value. This patch uses in all
cases an offset, which is the result of a keyed hash function taking
the source and destination addresses and port numbers into account.
The keyed hash function is the same a used for the initial TSN.

Reviewed by: rrs@
MFC after: 1 month
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16636