virtio-scsi: fix use-after-free of VirtIOSCSIReq

scsi_req_continue can complete the request and cause the VirtIOSCSIReq
to be freed.  Fetch req->sreq just once to avoid the bug.

Reported-by: Richard Jones <rjones@redhat.com>
Tested-by: Richard Jones <rjones@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 203e624..6c02fe2 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -545,11 +545,12 @@
 
 void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req)
 {
-    if (scsi_req_enqueue(req->sreq)) {
-        scsi_req_continue(req->sreq);
+    SCSIRequest *sreq = req->sreq;
+    if (scsi_req_enqueue(sreq)) {
+        scsi_req_continue(sreq);
     }
-    bdrv_io_unplug(req->sreq->dev->conf.bs);
-    scsi_req_unref(req->sreq);
+    bdrv_io_unplug(sreq->dev->conf.bs);
+    scsi_req_unref(sreq);
 }
 
 static void virtio_scsi_handle_cmd(VirtIODevice *vdev, VirtQueue *vq)