coda: cleanup /dev/cfs open and close handling

- Make sure device index is not a negative number.
- Unlink queued requests when the device is closed to avoid passing them
  to the next opener.

Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>