virtio: return correct capacity to users

We can't rely on indirect buffers for capacity
calculations because they need a memory allocation
which might fail.  In particular, virtio_net can get
into this situation under stress, and it drops packets
and performs badly.

So return the number of buffers we can guarantee users.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reported-By: Krishna Kumar2 <krkumar2@in.ibm.com>

drivers/virtio/virtio_ring.c

@@ -230,9 +230,6 @@ add_head:
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
 
-	/* If we're indirect, we can fit many (assuming not OOM). */
-	if (vq->indirect)
-		return vq->num_free ? vq->vring.num : 0;
 	return vq->num_free;
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_buf_gfp);