* TCP segments without timestamps should be dropped when support for
  the timestamp option has been negotiated.
* TCP segments with timestamps should be processed normally if support
  for the timestamp option has not been negotiated.
This patch enforces the above.

PR:			250499
Reviewed by:		gnn, rrs
MFC after:		1 week
Sponsored by:		Netflix, Inc
Differential Revision:	https://reviews.freebsd.org/D27148