MFC r335194:
When retransmitting TCP SYN-ACK segments with the TCP timestamp option
enabled use an updated timestamp instead of reusing the one used in
the initial TCP SYN-ACK segment.

This patch ensures that an updated timestamp is used when sending the
SYN-ACK from the syncache code. It was already done if the
SYN-ACK was retransmitted from the generic code.

This makes the behaviour consistent and also conformant with
the TCP specification.

Reviewed by: jtl@, Jason Eggleston
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D15634