librbd: delay commit of overwritten journal event

librbd: delay commit of overwritten journal event

With the cache enabled and write-after-write IOs to the same
object extents, it was possible for the overwritten journal event
to be committed before the overwriter journal event was written
to disk.  If a client crash occurs before the event is written,
the image will be inconsistent on replay.

Fixes: http://tracker.ceph.com/issues/15938
Signed-off-by: Jason Dillaman <dillaman@redhat.com>