zebra: Properly note that a nhg's nexthop has gone down

zebra: Properly note that a nhg's nexthop has gone down

Current code when a link is set down is to just mark the
nexthop group as not properly setup.  Leaving situations
where when an interface goes down and show output is
entered we see incorrect state.  This is true for anything
that would be checking those flags at that point in time.

Modify the interface down nexthop group code to notice the
nexthops appropriately ( and I mean set the appropriate flags )
and to allow a `show ip route` command to actually display
what is going on with the nexthops.

eva# show ip route 1.0.0.0
Routing entry for 1.0.0.0/32
  Known via "sharp", distance 150, metric 0, best
  Last update 00:00:06 ago
  * 192.168.44.33, via dummy1, weight 1
  * 192.168.45.33, via dummy2, weight 1

sharpd@eva:~/frr1$ sudo ip link set dummy2 down

eva# show ip route 1.0.0.0
Routing entry for 1.0.0.0/32
  Known via "sharp", distance 150, metric 0, best
  Last update 00:00:12 ago
  * 192.168.44.33, via dummy1, weight 1
    192.168.45.33, via dummy2 inactive, weight 1

Notice now that the 1.0.0.0/32 route now correctly
displays the route for the nexthop group entry.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit 266b0619942edd8d838235e14dc0cb71a772f585)