hw/nvme: fix memory leak in nvme_dsm

The iocb (and the allocated memory to hold LBA ranges) leaks if reading
the LBA ranges fails.

Fix this by adding a free and an unref of the iocb.

Reported-by: Coverity (CID 1508281)
Fixes: d7d1474f ("hw/nvme: reimplement dsm to allow cancellation")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>