vhost/scsi: Use safe iteration in vhost_scsi_complete_cmd_work()

The following patch changed the behavior which originally did safe
iteration. Make it safe as it was.

   12bdcbd5
   vhost/scsi: Don't reinvent the wheel but use existing llist API

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>