librbd: prevent concurrent AIO callbacks to external clients

librbd: prevent concurrent AIO callbacks to external clients

With the various threads and conditional IO paths within librbd, it was
possible for external AIO callbacks to be concurrently executed from
different librbd threads. These callbacks should be serialized to reduce
the unexpected potential for data corruption.

Fixes: http://tracker.ceph.com/issues/40417
Signed-off-by: Jason Dillaman <dillaman@redhat.com>