kern/41511: vnconfig fails on second use

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

>Number:         41511
>Category:       kern
>Synopsis:       vnconfig fails on second use
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat May 30 14:10:00 +0000 2009
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2009.05.28.10.44.29
>Organization:
>Environment:
System: NetBSD i386 on qemu
Architecture: i386
Machine: i386
>Description:

Repatedly configuring and unconfiguring a vnd device on -current
causes strange warning messages ("module error") and finally hangs.

>How-To-Repeat:

Run the following sequence of commands on a recent -current:

dd if=/dev/zero of=/tmp/img bs=1k count=1024
vnconfig /dev/vnd0 /tmp/img
vnconfig -u /dev/vnd0
vnconfig /dev/vnd0 /tmp/img
vnconfig -u /dev/vnd0

Observe that the second vnconfig command prints the message

  WARNING: module error: modcmd function returned error 17 for `vnd'
  vnconfig: /dev/vnd0d: opendisk: Device not configured

and the second "vnconfig -u" command hangs (but is interruptible
by SIGINT).

>Fix: