kern/41089: can't remount root read-only in NetBSD 5.0

[eravin%panix.com@localhost]

>Number:         41089
>Category:       kern
>Synopsis:       can't remount root read-only in NetBSD 5.0
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 28 19:15:00 +0000 2009
>Originator:     Ed Ravin
>Release:        5.0_RC2
>Organization:
Public Access Networks
>Environment:
NetBSD testing.panix.com 5.0_RC2 NetBSD 5.0_RC2 (PANIX-XEN3U-USER-pae) #1: Sat 
Feb 21 20:24:11 EST 2009  
root%juggler.panix.com@localhost:/misc1/obj/misc2/devel/netbsd/5.0-RC2/src/sys/arch/i386/compile/PANIX-XEN3U-USER-pae
 i386
>Description:
We maintained a read-only root directory on a few NetBSD 4.0 systems.

After upgrading to NetBSD 5.0 betas and release candidates, we
can boot with the read-only root, but if we remount it r/w and then
try to remount it read-only, we get an error:

   root@testing ~: # mount -u -o ro /
   mount_ffs: /rdev/xbd0a on /: Operation not supported

The symptoms are very similar to kern/13574 but I'm opening a new bug because 
that was opened back in 2001 against NetBSD 1.5 and we've been doing this for 
years with NetBSD 4.0, 3.0 and 1.5 with no problems.
>How-To-Repeat:

>Fix: