kern/53928: modules/t_builtin:disable test case randomly fails

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

>Number:         53928
>Category:       kern
>Synopsis:       modules/t_builtin:disable 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 Jan 30 08:15:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:

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

The "disable" test case of the modules/t_builtin test program randomly
fails with the error message "rump_sys_modctl(MODCTL_UNLOAD, kernfs):
Device busy".

For example, in the i386 tests on the TNF testbed, 78 runs out of 227
have failed so far this year.  Log output from the most recent failure
is at:

  http://releng.netbsd.org/b5reports/i386/2019/2019.01.29.12.01.20/test.html#modules_t_builtin_disable

The test did not fail in 2015 or 2016, and failed for the first
time with source date 2017.05.08.06.39.23.

It is also failing on real amd64 hardware, for example:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2019/2019.01.30.01.20.47/test.html#modules_t_builtin_disable

>How-To-Repeat:

Run the ATF tests repeatedly.

>Fix: