selftests: ublk: fix write cache implementation

For loop target, write cache isn't enabled, and each write isn't be
marked as DSYNC too.

Fix it by enabling write cache, meantime fix FLUSH implementation
by not taking LBA range into account, and there isn't such info
for FLUSH command.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250321004758.152572-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/file_backed.c

@@ -123,10 +123,7 @@ static int loop_queue_tgt_io(struct ublk_queue *q, int tag)
 		sqe = ublk_queue_alloc_sqe(q);
 		if (!sqe)
 			return -ENOMEM;
-		io_uring_prep_sync_file_range(sqe, 1 /*fds[1]*/,
-				iod->nr_sectors << 9,
-				iod->start_sector << 9,
-				IORING_FSYNC_DATASYNC);
+		io_uring_prep_fsync(sqe, 1 /*fds[1]*/, IORING_FSYNC_DATASYNC);
 		io_uring_sqe_set_flags(sqe, IOSQE_FIXED_FILE);
 		q->io_inflight++;
 		sqe->user_data = build_user_data(tag, ublk_op, UBLK_IO_TGT_NORMAL, 1);
@@ -187,6 +184,7 @@ static int ublk_loop_tgt_init(struct ublk_dev *dev)
 	struct ublk_params p = {
 		.types = UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DMA_ALIGN,
 		.basic = {
+			.attrs = UBLK_ATTR_VOLATILE_CACHE,
 			.logical_bs_shift	= 9,
 			.physical_bs_shift	= 12,
 			.io_opt_shift	= 12,