Cherry-picked from HEAD:

Notes:
    svn path=/head/; revision=273014

A connection in TIME_WAIT state before calling close() actually did not
received any RST packet.  Do not set error to ECONNRESET in this case.

Differential Revision:	https://reviews.freebsd.org/D879
Reviewed by:		rpaulo, adrian
Approved by:		jhb (mentor)
Sponsored by:		Verisign, Inc.
Cherry-picked from HEAD:

Notes:
    svn path=/head/; revision=273850

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

Conflicts:
	sys/netinet/tcp_timewait.c
	sys/netinet/tcp_var.h