bin/55040: mount(8) output leaks path from chroot

[roy%marples.name@localhost]

>Number:         55040
>Category:       bin
>Synopsis:       mount(8) output leaks path from chroot
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Mar 01 21:40:00 +0000 2020
>Originator:     Roy Marples
>Release:        9.99.49
>Organization:
>Environment:
$ uname -a
NetBSD cube.marples.name 9.99.48 NetBSD 9.99.48 (GENERIC) #2: Sun Mar  1 15:49:06 UTC 2020  roy%cube.marples.name@localhost:/usr/obj/sys/arch/amd64/compile.amd64/GENERIC amd64
>Description:
It should be noted that the Root on ZFS ramdisk is in use.
/ is FFS on a ramdisk
/altroot is ZFS mounted

init.root = /altroot

I *think* mount output was correct from 9.99.47, but unsure.
>How-To-Repeat:
# cd /tmp
# mkdir test
# cd test
# for x in bin sbin lib libexec; do mkdir $x; mount -t null /$x $x; done
# chroot .
# mount
/bin on /altroot/tmp/x/bin type null (local)
/sbin on /altroot/tmp/x/sbin type null (local)
/lib on /altroot/tmp/x/lib type null (local)
/libexec on /altroot/tmp/x/libexec type null (local)

>Fix: