kern/54062: lib/libc/sys/t_sendmmsg/sendmmsg_basic test regression

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54062
>Category:       kern
>Synopsis:       lib/libc/sys/t_sendmmsg/sendmmsg_basic test regression
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 16 11:55:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The lib/libc/sys/t_sendmmsg/sendmmsg_basic test case has recently
started failing in every run.

It was previously passing reliably on my testbed on real amd64
hardware, and passing most of the time on the qemu-based TNF i386
testbed.

The problem started during the recent period when the tests were
panicing the system due to pmap bugs, which makes it harder to pin
down the offending commit.  The first failure recorded on b5 is here:

  http://releng.netbsd.org/b5reports/i386/commits-2019.03.html#2019.03.12.11.05.09

>How-To-Repeat:

>Fix: