kern/52598: /proc/self/exe misbehaves if it's on a null mount

[coypu%sdf.org@localhost]

>Number:         52598
>Category:       kern
>Synopsis:       /proc/self/exe misbehaves if it's on a null mount
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Oct 07 09:05:00 +0000 2017
>Originator:     coypu
>Release:        NetBSD 8.99.2
>Organization:
>Environment:
NetBSD localhost 8.99.2 NetBSD 8.99.2 (GENERIC) #1: Fri Sep  1 02:38:58 IDT 2017  fly@loggy:/home/fly/x86/sys/arch/amd64/compile/GENERIC amd64

>Description:
# mount -t null /rescue /mnt
# /rescue/ls -l /proc/self/exe
lr-xr-xr-x  1 root  wheel  10 Oct  7 11:55 /proc/self/exe -> /rescue/ls
# /mnt/ls -l /proc/self/exe
lr-xr-xr-x  1 root  wheel  1 Oct  7 11:55 /proc/self/exe -> /

>How-To-Repeat:

>Fix: