blackhole_netdev: use blackhole_netdev to invalidate dst entries

Use blackhole_netdev instead of 'lo' device with lower MTU when marking
dst "dead".

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Tested-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>