vhost: make msg padding explicit

There's a 32 bit hole just after type. It's best to
give it a name, this way compiler is forced to initialize
it with rest of the structure.
Reported-by: NKevin Easton <kevin@guarana.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/vhost.h

@@ -68,6 +68,7 @@ struct vhost_iotlb_msg {
 
 struct vhost_msg {
 	int type;
+	int padding0;
 	union {
 		struct vhost_iotlb_msg iotlb;
 		__u8 padding[64];