block/nbd-client: assert qiov len once in nbd_co_request

Also improve the assertion: check that qiov is NULL for other commands
than CMD_READ and CMD_WRITE.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171012095319.136610-2-vsementsov@virtuozzo.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

block/nbd-client.c

@@ -156,7 +156,6 @@ static int nbd_co_send_request(BlockDriverState *bs,
         qio_channel_set_cork(s->ioc, true);
         rc = nbd_send_request(s->ioc, request);
         if (rc >= 0 && !s->quit) {
-            assert(request->len == iov_size(qiov->iov, qiov->niov));
             if (qio_channel_writev_all(s->ioc, qiov->iov, qiov->niov,
                                        NULL) < 0) {
                 rc = -EIO;
@@ -197,7 +196,6 @@ static int nbd_co_receive_reply(NBDClientSession *s,
         assert(s->reply.handle == request->handle);
         ret = -s->reply.error;
         if (qiov && s->reply.error == 0) {
-            assert(request->len == iov_size(qiov->iov, qiov->niov));
             if (qio_channel_readv_all(s->ioc, qiov->iov, qiov->niov,
                                       NULL) < 0) {
                 ret = -EIO;
@@ -231,8 +229,12 @@ static int nbd_co_request(BlockDriverState *bs,
     NBDClientSession *client = nbd_get_client_session(bs);
     int ret;
 
-    assert(!qiov || request->type == NBD_CMD_WRITE ||
-           request->type == NBD_CMD_READ);
+    if (qiov) {
+        assert(request->type == NBD_CMD_WRITE || request->type == NBD_CMD_READ);
+        assert(request->len == iov_size(qiov->iov, qiov->niov));
+    } else {
+        assert(request->type != NBD_CMD_WRITE && request->type != NBD_CMD_READ);
+    }
     ret = nbd_co_send_request(bs, request,
                               request->type == NBD_CMD_WRITE ? qiov : NULL);
     if (ret < 0) {