rtnetlink: add new value for DHCP added routes

To improve manageability, it would be good to be able to disambiguate routes
added by administrator from those added by DHCP client.  The only necessary
kernel change is to add value to rtnetlink include file so iproute2 utility
can use it.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>