virtio: fix vring_init for 64 bits

This patch fixes a typo in vring_init().  This happens to work today in lguest
because the sizeof(struct vring_desc) is 16 and struct vring contains 3
pointers and an unsigned int so on 32-bit
sizeof(struct vring_desc) == sizeof(struct vring).  However, this is no longer
true on 64-bit where the bug is exposed.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_ring.h

@@ -92,8 +92,8 @@ static inline void vring_init(struct vring *vr, unsigned int num, void *p)
 {
 	vr->num = num;
 	vr->desc = p;
-	vr->avail = p + num*sizeof(struct vring);
-	vr->used = p + (num+1)*(sizeof(struct vring) + sizeof(__u16));
+	vr->avail = p + num*sizeof(struct vring_desc);
+	vr->used = p + (num+1)*(sizeof(struct vring_desc) + sizeof(__u16));
 }
 
 static inline unsigned vring_size(unsigned int num)