integrity: use audit_log_string

Based on a request from Eric Paris to simplify parsing, replace
audit_log_format statements containing "%s" with audit_log_string().

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>