dm writecache: reject asynchronous pmem devices

commit a46624580376a3a0beb218d94cbc7f258696e29f upstream.

DM writecache does not handle asynchronous pmem. Reject it when
supplied as cache.

Link: https://lore.kernel.org/linux-nvdimm/87lfk5hahc.fsf@linux.ibm.com/


Fixes: 6e84200c ("virtio-pmem: Add virtio pmem driver")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org # 5.3+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>