umount -f broken again [with devfs/NFS]

[Garrett Cooper]

    Ran into this with a build of CURRENT 2 week or so old version of
CURRENT (gjb's livecd) and while unmounting devfs with a week old
build:

# umount -f /ifs
umount: unmount of /ifs failed: Device busy
# uname -a
FreeBSD wf158.west.isilon.com 10.0-CURRENT FreeBSD 10.0-CURRENT #3
r+5a05236: Wed Dec 12 17:35:14 PST 2012
root at wf158.west.isilon.com:/usr/obj/usr/src/sys/ISI-GENERIC  amd64

    My gut reaction is that some recently changes to VFS probably
broke this (a couple week old 9-STABLE build just hangs on an NFS
mountpoint that's gone out to lunch).
Thanks,
-Garrett