virtio: make add_buf return capacity remaining

This API change means that virtio_net can tell how much capacity
remains for buffers.  It's necessarily fuzzy, since
VIRTIO_RING_F_INDIRECT_DESC means we can fit any number of descriptors
in one, *if* we can kmalloc.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Dinesh Subhraveti <dineshs@us.ibm.com>

drivers/char/hw_random/virtio-rng.c

@@ -51,7 +51,7 @@ static void register_buffer(void)
 
 	sg_init_one(&sg, random_data+data_left, RANDOM_DATA_SIZE-data_left);
 	/* There should always be room for one buffer. */
-	if (vq->vq_ops->add_buf(vq, &sg, 0, 1, random_data) != 0)
+	if (vq->vq_ops->add_buf(vq, &sg, 0, 1, random_data) < 0)
 		BUG();
 	vq->vq_ops->kick(vq);
 }