vhost: disable metadata prefetch optimization

This seems to cause guest and host memory corruption.
Disable for now until we get a better handle on that.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.h

@@ -96,7 +96,7 @@ struct vhost_uaddr {
 };
 
 #if defined(CONFIG_MMU_NOTIFIER) && ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE == 0
-#define VHOST_ARCH_CAN_ACCEL_UACCESS 1
+#define VHOST_ARCH_CAN_ACCEL_UACCESS 0
 #else
 #define VHOST_ARCH_CAN_ACCEL_UACCESS 0
 #endif