virtio_pmem: Check device status before requesting flush

In the Linux kernel, the following vulnerability has been resolved: virtio_pmem: Check device status before requesting flush If a pmem device is in a bad status, the driver side could wait for host ack forever in virtio_pmem_flush(), causing the system to hang. So add a status check in the beginning of virtio_pmem_flush() to return early if the device is not activated.