IMA: do not measure everything opened by root by default

The IMA default policy measures every single file opened by root.  This is
terrible for most users.  Consider a system (like mine) with virtual machine
images.  When those images are touched (which happens at boot for me) those
images are measured.  This is just way too much for the default case.

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