IMA: drop the word integrity in the audit message

integrity_audit_msg() uses "integrity:" in the audit message.  This
violates the (loosely defined) audit system requirements that everything be
a key=value pair and it doesn't provide additional information.  This can
be obviously gleaned from the message type.  Just drop it.

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