do 'ip route replace default' instead of 'add'

do 'ip route replace default' instead of 'add'

When you define the gateway for an interface, dracut sets it up with:

  ip route add default via $gw dev $netif

If a default route is already set (e.g. if you have multiple NICs), this
will fail with the message "RTNETLINK answers: File exists".

So, if your first NIC isn't usable as a default route

Using "ip route replace default" instead allows ifup/dhclient-script to
correctly change the default route to the new interface.