CRED: Fix memory leak in error handling

Fix a memory leak on an OOM condition in prepare_usermodehelper_creds().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>