Add "scope link" when adding routes without a nexthop.

This is consistent with what /sbin/ip does, and it makes
recursive lookups on secondary tables work even when the main
table is empty.

This was originally proposed by Sreeram as http://ag/506456 to
fix a VPN issue, but then abandoned because it did not fix that
particular problem.

Bug: 16628572
Change-Id: I85753389c683ae2127b21af722463a35a33b60eb