When retransmitting TCP SYN-ACK segments with the TCP timestamp option

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
MFC after:		1 month
Sponsored by:		Neflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D15634