rbd: introduce a per-device ordered workqueue

This is going to be used for re-registering watch requests and
exclusive-lock tasks: acquire/request lock, notify-acquired, release
lock, notify-released.  Some refactoring in the map/unmap paths was
necessary to give this workqueue a meaningful name: "rbdX-tasks".

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>