MFC r348290:
When an ACK segment as the third message of the three way handshake is
received and support for time stamps was negotiated in the SYN/SYNACK
exchange, perform the PAWS check and only expand the syn cache entry if
the check is passed.
Without this check, endpoints may get stuck on the incomplete queue.

Reviewed by: jtl@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D20374