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>