IMA: add policy rule to measure critical data

A new IMA policy rule is needed for the IMA hook
ima_measure_critical_data() and the corresponding func CRITICAL_DATA for
measuring the input buffer.  The policy rule should ensure the buffer
would get measured only when the policy rule allows the action.  The
policy rule should also support the necessary constraints (flags etc.)
for integrity critical buffer data measurements.

Add policy rule support for measuring 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>