[sheepdog-users] ssd cache re-enable with cause data lost

[redtone]

1, run sheep with ssd cache before one week, (-w size=20000,dir=/ssdcache)
2, run sheep with out ssd cache for one week.
3, run sheep with ssd cache today with the same ¨Cw option.

Then I found the VMs disk data is lost. 

So I check the VM¡¯s disk size. I found the vdi¡¯s size is 276M, but in fact
it should be 7GB.uploaded by user in the week.

I¡¡think when I re-enable the ssd cache, the old data in the cache is used
and overwrite the new data.

Please check the cache¡¯s work flow to fixed the bug