osd: set the blocked_by relationship when rolling back to a degraded

osd: set the blocked_by relationship when rolling back to a degraded
object

In a scenario like below:
- A rollback op comes in, and is enqueued.
- Several other ops on the same object come in, and are enqueued.
- The rollback op dispatches, and finds the object which it rollbacks to is
degraded, then this op is pushbacked into a list to wait for the degraded
object to recover.
- The later ops are handled and responded back to client.
- The degraded object recovers. The rollback op is enqueued again and finally
responded to client.
This breaks the op order. Need to set the blocked_by relationship to enqueue
the later ops until the degraded object recovers.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>