hw/nvme: fix memory leak in nvme_dsm

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: d7d1474fd85d ("hw/nvme: reimplement dsm to allow cancellation")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
(cherry picked from commit 4b32319cdacd99be983e1a74128289ef52c5964e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>