ima: add dont_audit action to suppress audit actions

ima: add dont_audit action to suppress audit actions

"measure", "appraise" and "hash" actions all have corresponding "dont_*"
actions, but "audit" currently lacks that. This means it is not
currently possible to have a policy that audits everything by default,
but excludes specific cases.

This seems to have been an oversight back when the "audit" action was
added.

Add a corresponding "dont_audit" action to enable such uses.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>