xen-blkback: really don't leak mode property

Commit 9d092603 ("xen-blkback: do not leak mode property") left one
path unfixed; correct this.

Acked-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>