RFC 7323 specifies that:

• 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