IMA: define a hook to measure kernel integrity critical data

IMA provides capabilities to measure file and buffer data.  However,
various data structures, policies, and states stored in kernel memory
also impact the integrity of the system.  Several kernel subsystems
contain such integrity critical data.  These kernel subsystems help
protect the integrity of the system.  Currently, IMA does not provide a
generic function for measuring kernel integrity critical data.

Define ima_measure_critical_data, a new IMA hook, to measure kernel
integrity critical data.

Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>