FROMGIT: 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.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>

Bug: 358391069
Change-Id: I325e6f0ea047c4c5fa82cf4b590cbf7240f39b7b
(cherry picked from commit e25fbcd9 https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git

 master)
Signed-off-by: Philip Chen <philipchen@google.com>