net: dsa: Add netconsole support

Add support for using DSA slave network devices with netconsole, which
requires us to allocate and free custom netpoll instances and invoke the
parent network device poll controller callback.

In order for netconsole to work, we need to construct the DSA tag, but
not queue the skb for transmission on the master network device xmit
function.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>