coda: ignore returned values when upcalls return errors

Venus returns an ENOENT error on open, so we shouldn't try to grab the
filehandle for the returned fd.

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>