IMA: Minimal IMA policy and boot param for TCB IMA policy

The IMA TCB policy is dangerous.  A normal use can use all of a system's
memory (which cannot be freed) simply by building and running lots of
executables.  The TCB policy is also nearly useless because logging in as root
often causes a policy violation when dealing with utmp, thus rendering the
measurements meaningless.

There is no good fix for this in the kernel.  A full TCB policy would need to
be loaded in userspace using LSM rule matching to get both a protected and
useful system.  But, if too little is measured before userspace can load a real
policy one again ends up with a meaningless set of measurements.  One option
would be to put the policy load inside the initrd in order to get it early
enough in the boot sequence to be useful, but this runs into trouble with the
LSM.  For IMA to measure the LSM policy and the LSM policy loading mechanism
it needs rules to do so, but we already talked about problems with defaulting
to such broad rules....

IMA also depends on the files being measured to be on an FS which implements
and supports i_version.  Since the only FS with this support (ext4) doesn't
even use it by default it seems silly to have any IMA rules by default.

This should reduce the performance overhead of IMA to near 0 while still
letting users who choose to configure their machine as such to inclue the
ima_tcb kernel paramenter and get measurements during boot before they can
load a customized, reasonable policy in userspace.

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