virtio: return ENOMEM on out of memory

add_buf returns ring size on out of memory,
this is not what devices expect.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org # .34.x

drivers/virtio/virtio_ring.c

@@ -119,7 +119,7 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 
 	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
 	if (!desc)
-		return vq->vring.num;
+		return -ENOMEM;
 
 	/* Transfer entries from the sg list into the indirect page */
 	for (i = 0; i < out; i++) {