hw/nvme: add missing return statement

hw/nvme: add missing return statement

Since there is no return after nvme_dsm_cb invocation, metadata
associated with non-zero block range is currently zeroed. Also this
behaviour leads to segfault since we schedule iocb->bh two times.
First when entering nvme_dsm_cb with iocb->idx == iocb->nr and
second because of missing return on call stack unwinding by calling
blk_aio_pwrite_zeroes and subsequent nvme_dsm_cb callback.

Fixes: d7d1474fd85d ("hw/nvme: reimplement dsm to allow cancellation")
Signed-off-by: Dmitry Tikhov <d.tihov@yadro.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>