vhost: correctly check the return value of translate_desc() in log_used()

[ Upstream commit 816db7663565cd23f74ed3d5c9240522e3fb0dda ]

When fail, translate_desc() returns negative value, otherwise the
number of iovs. So we should fail when the return value is negative
instead of a blindly check against zero.

Detected by CoverityScan, CID# 1442593:  Control flow issues  (DEADCODE)

Fixes: cc5e71075947 ("vhost: log dirty page correctly")
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Reported-by: NStephen Hemminger <stephen@networkplumber.org>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>

drivers/vhost/vhost.c

@@ -1784,7 +1784,7 @@ static int log_used(struct vhost_virtqueue *vq, u64 used_offset, u64 len)
 
 	ret = translate_desc(vq, (uintptr_t)vq->used + used_offset,
 			     len, iov, 64, VHOST_ACCESS_WO);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
 	for (i = 0; i < ret; i++) {