librados: fix locking for AioCompletionImpl refcounting

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>