block/nbd-client: nbd_co_send_request: fix return code

It's incorrect to return success rc >= 0 if we skip qio_channel_writev_all()
call due to s->quit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170920124507.18841-4-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd-client.c

@@ -161,6 +161,8 @@ static int nbd_co_send_request(BlockDriverState *bs,
                                        NULL) < 0) {
                 rc = -EIO;
             }
+        } else if (rc >= 0) {
+            rc = -EIO;
         }
         qio_channel_set_cork(s->ioc, false);
     } else {