vhost: vringh: Fix copy_to_iter return value check

The return value of copy_to_iter can't be negative, check whether the
copied length is equal to the requested length instead of checking for
negative values.

Fixes: 309bba39c945 ("vringh: iterate on iotlb_translate to handle large translations")
Cc: "Stefano Garzarella" <[e-mail]>
Cc: zhang jiao <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]
Message-ID: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vhost/vringh.c

···
 		struct iov_iter iter;
 		u64 translated;
 		int ret;
+		size_t size;
 
 		ret = iotlb_translate(vrh, (u64)(uintptr_t)dst,
 				      len - total_translated, &translated,
···
 				      translated);
 		}
 
-		ret = copy_to_iter(src, translated, &iter);
-		if (ret < 0)
-			return ret;
+		size = copy_to_iter(src, translated, &iter);
+		if (size != translated)
+			return -EFAULT;
 
 		src += translated;
 		dst += translated;