rbd: Skip rbd cache flush if journaling is enabled under aio_flush

rbd: Skip rbd cache flush if journaling is enabled under aio_flush

With journaling rbd writes will be persisteted on rbd journal objects.
The journal will be replayed if crash happen. So it's not necessary to
flush rbd_cache in this case. This will improve the flush latency.

This patch adds checking on handling aio_flush: if journaling is
enabled, rbd cache flushing is skipped.
In a system flush(ImageCtx::flush) the cache is flushed even w/ journaling
where we truly do need to flush all IO out to disk.

Fixes: http://tracker.ceph.com/issues/15761
Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>