loopback: create blackhole net device similar to loopack.

Create a blackhole net device that can be used for "dead"
dst entries instead of loopback device. This blackhole device differs
from loopback in few aspects: (a) It's not per-ns. (b)  MTU on this
device is ETH_MIN_MTU (c) The xmit function is essentially kfree_skb().
and (d) since it's not registered it won't have ifindex.

Lower MTU effectively make the device not pass the MTU check during
the route check when a dst associated with the skb is dead.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>