nbd: check sock index in nbd_read_stat()

The sock that clent send request in nbd_send_cmd() and receive reply
in nbd_read_stat() should be the same.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210916093350.1410403-4-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>