port-pmax/55610: pmax testbed panics with "address error"

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

>Number:         55610
>Category:       port-pmax
>Synopsis:       pmax testbed panics with "address error"
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    port-pmax-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Aug 27 11:10:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.07.23.15.24.37
>Organization:
>Environment:
System: NetBSD
Architecture: mips
Machine: pmax
>Description:

>How-To-Repeat:

The TNF NetBSD/pmax testbed consistently panics during the
closed_slave test case of the t_ttypty test:

  kernel/kqueue/read/t_ttypty (68/875): 3 test cases
      closed_slave: [ 2970.8125030] pid 22491(t_ttypty): trap: cpu0, address error (load or I-fetch) in kernel mode
  [ 2970.8125030] status=0x8ff34, cause=0x10, epc=0x8016b0f8, vaddr=0xfe6
  [ 2970.8125030] tf=0x808a99f0 ksp=0x808a9a90 ra=0x8016b0d0 ppl=0
  [ 2970.8125030] kernel: address error (load or I-fetch) trap
  Stopped in pid 22491.22491 (t_ttypty) at	netbsd:ptyfs_get_node+0x90:
  lw	v0,8(s0)
  db>

The problem started with this commit:

  2020.07.23.12.15.59 skrll src/sys/arch/mips/include/mips_param.h 1.42

Logs:

  http://releng.netbsd.org/b5reports/pmax/commits-2020.07.html#2020.07.23.12.15.59

>Fix: