[PATCH] iseries_veth: Cleanup skbs to prevent unregister_netdevice() hanging

Hi Andrew, Jeff,

The iseries_veth driver is badly behaved in that it will keep TX packets
hanging around forever if they're not ACK'ed and the queue never fills up.

This causes the unregister_netdevice code to wait forever when we try to take
the device down, because there's still skbs around with references to our
struct net_device.

There's already code to cleanup any un-ACK'ed packets in veth_stop_connection()
but it's being called after we unregister the net_device, which is too late.

The fix is to rearrange the module exit function so that we cleanup any
outstanding skbs and then unregister the driver.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>