librados: fix locking for AioCompletionImpl refcounting

Add an already-locked helper so that C_Aio{Safe,Complete} can
increment the reference count when their caller holds the
lock. C_AioCompleteAndSafe's caller is not holding the lock, so call
regular get() to ensure no racing updates can occur.

This eliminates all direct manipulations of AioCompletionImpl->ref,
and makes the necessary locking clear.

The only place C_AioCompleteAndSafe is used is in handling
aio_flush_async(). This could cause a missing completion.

Refs: #5919
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Tested-by: Oliver Francke <Oliver.Francke@filoo.de>
librados: fix async aio completion wakeup

For aio flush, we register a wait on the most recent write.  The write
completion code, however, was *only* waking the waiter if they were waiting
on that write, without regard to previous writes (completed or not).
For example, we might have 6 and 7 outstanding and wait on 7.  If they
finish in order all is well, but if 7 finishes first we do the flush
completion early.  Similarly, if we

 - start 6
 - start 7
 - finish 7
 - flush; wait on 7
 - finish 6

we can hang forever.

Fix by doing any completions that are prior to the oldest pending write in
the aio write completion handler.

Refs: #5919

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Tested-by: Oliver Francke <Oliver.Francke@filoo.de>