audit: fix the RCU locking for the auditd_connection structure

Cong Wang correctly pointed out that the RCU read locking of the
auditd_connection struct was wrong, this patch correct this by
adopting a more traditional, and correct RCU locking model.

This patch is heavily based on an earlier prototype by Cong Wang.

Cc: <stable@vger.kernel.org> # 4.11.x-
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>