coda: fix creds reference

Needs a header file for credentials struct:

linux-next-20081023/fs/coda/file.c:177: error: dereferencing pointer to incomplete type

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>