ima: return IMA digest value only when IMA_COLLECTED flag is set

The IMA_COLLECTED flag indicates whether the IMA subsystem has
successfully collected a measurement for a given file object. Ensure
that we return the respective digest value stored within the iint
entry only when this flag has been set.

Failing to check for the presence of this flag exposes consumers of
this IMA API to receive potentially undesired IMA digest values when
an erroneous condition has been experienced in some of the lower level
IMA API code.

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>