ima: Use audit_log_format() rather than audit_log_string()

Remove the usage of audit_log_string() and replace it with
audit_log_format().

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Suggested-by: Steve Grubb <sgrubb@redhat.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>