Merge branch 'ima-buffer-measurement-changes-v4' into next-integrity

Based on cover letter "ima: Provide more info about buffer measurement":

Providing more information about buffer measurements requires
modification of the existing functions ima_measure_critical_data()
and process_buffer_measurement().  Unlike for file measurements,
there is no integrity_iint_cache structure for storing and retrieving
buffer measurements.

With these changes, kernel subsystems relying on IMA to measure
critical data don't have to duplicate the function to calculate the
buffer digest but, instead, can get directly the one that IMA would
insert, depending on the policy, in the new measurement list entry.