Staging: Fixed pohmelfs regression because of per-bdi writeback.

Since pohmelfs isn't tied to a single block device, it needs to setup a
backing dev like nfs/btrfs/etc do.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>