kern/56357: net/if_vlan/t_vlan:vlan_promisc test case randomly fails

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

>Number:         56357
>Category:       kern
>Synopsis:       net/if_vlan/t_vlan:vlan_promisc test case randomly fails
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Aug 11 07:55:00 +0000 2021
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
  
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

The vlan_promisc test case of the net/if_vlan/t_vlan test randomly
fails on some testbeds, including a physical Dell Latitude D400 laptop
running NetBSD/i386:

  https://www.gson.org/netbsd/bugs/build/i386-laptop/2021/2021.08.10.10.43.42/test.html#net_if_vlan_t_vlan_vlan_promisc

and the qemu-based TNF sparc testbed:

  http://releng.netbsd.org/b5reports/sparc/2021/2021.08.04.13.50.47/test.html#net_if_vlan_t_vlan_vlan_promisc

In each case, the error message is

  Fail: regexp PROMISC not in stdout

>How-To-Repeat:

On an affected system, run

  cd /usr/tests/net/if_vlan
  while atf-run t_vlan:vlan_promisc; do true; done

>Fix: