nbd: partition nbd_read_stat() into nbd_read_reply() and nbd_handle_reply()

Prepare to fix uaf in nbd_read_stat(), no functional changes.

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/20210916093350.1410403-7-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>