kern/55692: rump kernels randomly panic with "rn_init 2"

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

>Number:         55692
>Category:       kern
>Synopsis:       rump kernels randomly panic with "rn_init 2"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 02 13:05:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Running the ATF tests on real amd64 hardware, test cases sometimes
fail with a rump kernel issuing the message "panic: rn_init 2".
For example:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.04.27.20.46.01/test.html#net_ipsec_t_ipsec_l2tp_ipsec_l2tp_ipv4_tunnel_esp_null
  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.09.17.17.33.50/test.html#fs_nfs_t_mountd_mountdhup
  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.09.26.08.21.27/test.html#fs_nfs_t_rquotad_get_nfs_be_1_group

For some initial analysis by kre, see PR 55164 (that PR is about an
unrelated issue, but the present issue came up because it happened to
cause a related test to fail, confusing me as to whether the issue of
that PR had been fixed).

>How-To-Repeat:

>Fix: