kern/55211: fs/vfs/t_vnops:nfs_dir_rmdirdotdot test fails

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

>Number:         55211
>Category:       kern
>Synopsis:       fs/vfs/t_vnops:nfs_dir_rmdirdotdot test fails
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Apr 26 18:45:02 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.04.19.13.26.17
>Organization:

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

The fs/vfs/t_vnops:nfs_dir_rmdirdotdot test case is failing for me on
real amd64 hardware:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/commits-2020.04.html#2020.04.19.13.26.17

The problem started with these commits:

  2020.04.19.13.25.00 hannken src/sys/kern/vfs_vnode.c 1.121
  2020.04.19.13.26.17 hannken src/sys/kern/vfs_mount.c 1.79

I'm also seeing failures under "qemu-system-x86_64 -accel nvmm", but
not under plain qemu.

>How-To-Repeat:

>Fix: