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.

Cc: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Link: https://lore.kernel.org/all/20250910091739.2999-1-zhangjiao2@cmss.chinamobile.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Fixes: 309bba3 ("vringh: iterate on iotlb_translate to handle large translations")
Link: https://patch.msgid.link/cd637504a6e3967954a9e80fc1b75e8c0978087b.1758723310.git.mst@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mstsirkin

drivers/vhost/vringh.c

@@ -1161,6 +1161,7 @@ static inline int copy_to_iotlb(const struct vringh *vrh, void *dst,
 		struct iov_iter iter;
 		u64 translated;
 		int ret;
+		size_t size;
 
 		ret = iotlb_translate(vrh, (u64)(uintptr_t)dst,
 				      len - total_translated, &translated,
@@ -1178,9 +1179,9 @@ static inline int copy_to_iotlb(const struct vringh *vrh, void *dst,
 				      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;