virtio-net: Fix kick/wait logic

The virtnet_transmit() logic for waiting the packet to be transmitted is
reversed: we can't wait the packet to be transmitted if we didn't kick()
the ring yet. The vring_more_used() while loop logic is reversed also,
that explains why the code works today.

The current code risks trying to free a buffer from the used ring
when none was available, that will happen most times because KVM
doesn't handle the packet immediately on kick(). Luckily it was working
because it was unlikely to have a buffer still queued for transmit when
virtnet_transmit() was called.

Also, adds a BUG_ON() to vring_get_buf(), to catch cases where we try
to free a buffer from the used ring when there was none available.

Patch for Etherboot. gPXE has the same problem on the code, but I hadn't
a chance to test gPXE using virtio-net yet.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

src/drivers/net/virtio-net.c

    u32 id;
    int ret;
 
+   BUG_ON(!vring_more_used(queue_index));
+
    elem = &vr->used->ring[last_used_idx[queue_index] % vr->num];
    wmb();
    id = elem->id;
 
    vring_add_buf(TX_INDEX, 0, 0);
 
+   vring_kick(nic, TX_INDEX, 1);
+
    /*
     * http://www.etherboot.org/wiki/dev/devmanual
     *
     *    before returning from this routine"
     */
 
-   while (vring_more_used(TX_INDEX)) {
+   while (!vring_more_used(TX_INDEX)) {
            mb();
            udelay(10);
    }
 
-   vring_kick(nic, TX_INDEX, 1);
-
    /* free desc */
 
    (void)vring_get_buf(TX_INDEX, NULL);