Fix a race condition in TCP timewait between tcp_tw_2msl_reuse() and
tcp_tw_2msl_scan(). This race condition drives unplanned timewait
timeout cancellation. Also simplify implementation by holding inpcb
reference and removing tcptw reference counting.

Differential Revision: https://reviews.freebsd.org/D826
Submitted by: Marc De la Gueronniere <mdelagueronniere@verisign.com>
Submitted by: jch
Reviewed By: jhb (mentor), adrian, rwatson
Sponsored by: Verisign, Inc.
MFC after: 2 weeks
X-MFC-With: r264321