The following patches implement the badfs filesystem in order to support
generic forced unmount (umount -f).  Badfs was originally written by
Tigran Aivazian and is now being cleaned up by me in hopes of
(eventually) getting it merged into mainline.

Resources used on the filesystem that keep it busy and prevent it from
being unmounted are redirected to badfs inodes that return EIO for all
operations.

Summary of patches:
  Patch 1/3 - Sets an f_light flag between fget_light() and fput_light().
  Patch 2/3 - Adds the badfs filesystem and the main forced unmount code.
  Patch 3/3 - EIO all future RPC requests from NFS during forced unmount.

I've tested forced unmount on ext3 and NFS (including tests where the
client has lost contact with the server) with all the basic cases listed
in patch 2/3.

These patches apply to 2.6.13-rc3 and are also available at:
    http://developer.osdl.org/njw/forced-unmount/patches/2.6.13-rc3/2/

Any additional testing and comments would be appreciated.