zebra: Reinstall nexthop when interface comes back up

If a interface down event caused a nexthop group to remove
one of the entries in the kernel, have it be reinstalled
when the interface comes back up.  Mark the nexthop as
usable.

new behavior:
eva# show nexthop-group rib 181818168
ID: 181818168 (sharp)
     RefCnt: 1
     Uptime: 00:00:23
     VRF: default(bad-value)
     Valid, Installed
     Depends: (35) (38) (44) (51)
           via 192.168.99.33, dummy1 (vrf default), weight 1
           via 192.168.100.33, dummy2 (vrf default), weight 1
           via 192.168.101.33, dummy3 (vrf default), weight 1
           via 192.168.102.33, dummy4 (vrf default), weight 1
eva# conf
eva(config)# int dummy3
eva(config-if)# shut
eva(config-if)# do show nexthop-group rib 181818168
ID: 181818168 (sharp)
     RefCnt: 1
     Uptime: 00:00:44
     VRF: default(bad-value)
     Depends: (35) (38) (44) (51)
           via 192.168.99.33, dummy1 (vrf default), weight 1
           via 192.168.100.33, dummy2 (vrf default), weight 1
           via 192.168.101.33, dummy3 (vrf default) inactive, weight 1
           via 192.168.102.33, dummy4 (vrf default), weight 1
eva(config-if)# no shut
eva(config-if)# do show nexthop-group rib 181818168
ID: 181818168 (sharp)
     RefCnt: 1
     Uptime: 00:00:53
     VRF: default(bad-value)
     Valid, Installed
     Depends: (35) (38) (44) (51)
           via 192.168.99.33, dummy1 (vrf default), weight 1
           via 192.168.100.33, dummy2 (vrf default), weight 1
           via 192.168.101.33, dummy3 (vrf default), weight 1
           via 192.168.102.33, dummy4 (vrf default), weight 1
eva(config-if)# exit
eva(config)# exit
eva# exit
sharpd@eva ~/frr1 (master) [255]> ip nexthop show id 181818168
id 181818168 group 35/38/44/51 proto 194
sharpd@eva ~/frr1 (master)>

Signed-off-by: Donald Sharp <[email protected]>

zebra/zebra_nhg.c

@@ -3782,6 +3782,17 @@ void zebra_interface_nhg_reinstall(struct interface *ifp)
 			frr_each_safe (nhg_connected_tree,
 				       &rb_node_dep->nhe->nhg_dependents,
 				       rb_node_dependent) {
+				struct nexthop *nhop_dependent =
+					rb_node_dependent->nhe->nhg.nexthop;
+
+				while (nhop_dependent &&
+				       !nexthop_same(nhop_dependent, nh))
+					nhop_dependent = nhop_dependent->next;
+
+				if (nhop_dependent)
+					SET_FLAG(nhop_dependent->flags,
+						 NEXTHOP_FLAG_ACTIVE);
+
 				if (IS_ZEBRA_DEBUG_NHG)
 					zlog_debug("%s dependent nhe %pNG Setting Reinstall flag",
 						   __func__,