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: d7d1474fd8 ("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>

hw/nvme/ctrl.c

@@ -2372,6 +2372,7 @@ static void nvme_dsm_md_cb(void *opaque, int ret)
         }
 
         nvme_dsm_cb(iocb, 0);
+        return;
     }
 
     iocb->aiocb = blk_aio_pwrite_zeroes(ns->blkconf.blk, nvme_moff(ns, slba),