vhost-scsi: flush backend after device ioctls

In the Linux kernel, the following vulnerability has been resolved: vhost-scsi: flush backend after device ioctls vhost-scsi translates guest response descriptors into userspace iovecs when commands are submitted. Target-core completes those commands asynchronously, so VHOST_SET_MEM_TABLE can replace the memory table while an in-flight command still retains response iovecs translated through the old table. If the old mapping is reused after VHOST_SET_MEM_TABLE returns, command completion can write the response to an unrelated userspace object. Flush the vhost-scsi backend after vhost_dev_ioctl() handles a device ioctl. This waits for in-flight commands that can still use the old response iovecs before the ioctl returns.