librbd: update AioCompletion return value before evaluating pending count

librbd: update AioCompletion return value before evaluating pending count

If the pending count is decremented before the return value is updated,
there is a possibility of two ASIO threads concurrently decrementing the
pending count down from 2 -> 1 -> 0. In the second thread (the one that
performs the final decrement from 1 -> 0), it can finalize the completion
before the first thread has had a chance to update the return value.

Fixes: https://tracker.ceph.com/issues/47847
Signed-off-by: Jason Dillaman <dillaman@redhat.com>