audit: audit_dummy_context can be boolean

This patch makes audit_dummy_context return bool due to this
particular function only using either one or zero as its return
value.

No functional change.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
[PM: subject line tweak]
Signed-off-by: Paul Moore <pmoore@redhat.com>