Fix for kern/20700

[buhrow%lothlorien.nfbcal.org@localhost (Brian Buhrow)]

        Hello. I've written a patch which addresses this bug and submitted it 
to the pr.
If no one objects, I'll commit this patch in a few days and request a
pullup to the 5.x branch.  If there are objections, I'm happy to discuss
and/or incorporate a better fix.  This fix works quite well in my case.
        If you're interested in a greater analysis and discussion of
thisissue, see:
http://mail-index.NetBSD.org/tech-kern/2011/11/30/msg012025.html

-thanks
-Brian