ima: Add code comments to explain IMA iint cache atomic_flags

ima: Add code comments to explain IMA iint cache atomic_flags

Explain these atomic flags to improve code readability. For example, the
flag IMA_DIGSIG is to indicate we mustn't update a file's security.ima
on close because the file already has IMA signature. The code comments
for the first three flags come from commit 0d73a55208e9 ("ima:
re-introduce own integrity cache lock") with a minor tweak.

Signed-off-by: Coiby Xu <coxu@redhat.com>
[zohar@linux.ibm.com: remove duplicate "integrity violation", unnecessary commas]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>