audit: make sure we don't let the retry queue grow without bounds

The retry queue is intended to provide a temporary buffer in the case
of transient errors when communicating with auditd, it is not meant
as a long life queue, that functionality is provided by the hold
queue.

This patch fixes a problem identified by Seth where the retry queue
could grow uncontrollably if an auditd instance did not connect to
the kernel to drain the queues.  This commit fixes this by doing the
following:

* Make sure we always call auditd_reset() if we decide the connection
with audit is really dead.  There were some cases in
kauditd_hold_skb() where we did not reset the connection, this patch
relocates the reset calls to kauditd_thread() so all the error
conditions are caught and the connection reset.  As a side effect,
this means we could move auditd_reset() and get rid of the forward
definition at the top of kernel/audit.c.

* We never checked the status of the auditd connection when
processing the main audit queue which meant that the retry queue
could grow unchecked.  This patch adds a call to auditd_reset()
after the main queue has been processed if auditd is not connected,
the auditd_reset() call will make sure the retry and hold queues are
correctly managed/flushed so that the retry queue remains reasonable.

Cc: <stable@vger.kernel.org> # 4.10.x-: 5b52330b
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>