integrity: ima audit dentry_open failure

Until we start appraising measurements, the ima_path_check()
return code should always be 0.

- Update the ima_path_check() return code comment
- Instead of the pr_info, audit the dentry_open failure

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