net: track link status of ipv6 nexthops

Add support to track current link status of ipv6 nexthops to match
recent changes that added support for ipv4 nexthops.  This takes a
simple approach to track linkdown status for next-hops and simply
checks the dev for the dst entry and sets proper flags that to be used
in the netlink message.

v2: drop use of rt6i_nhflags since it is not needed right now

Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: Dinesh Dutt <ddutt@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>