cifs: after upcalling for krb5 creds, invalidate key rather than revoking it

Calling key_revoke here isn't ideal as further requests for the key will
end up returning -EKEYREVOKED until it gets purged from the cache. What we
really intend here is to force a new upcall on the next request_key.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>