zebra: Nexthops need to be ACTIVE in some cases

Currently if you have an interface down event, Zebra
sets the nexthop(s) as !ACTIVE that use it.  On
interface up events the singleton nexthops are not being
set as ACTIVE.  Due to timing events it is sometimes
possible to end up with a route that is using a singleton

Change singleton nexthops to set the nexthop to ACTIVE.
This will allow the nexthop to be reinstalled appropriately
as well.

I was able to easily reproduce this using sharpd since
it does not attempt to reinstall the routes when a interface
goes up/down.

Before:

D>* 10.0.0.0/32 [150/0] via 192.168.102.34, dummy2, weight 1, 00:00:01

sharpd@eva ~/frr5 (master)> sudo ip link set dummy2 down ; sudo ip link set dummy2 up

D>  10.0.0.0/32 [150/0] (350) via 192.168.102.34, dummy2 inactive, weight 1, 00:00:10

After code change:

D>* 10.0.0.0/32 [150/0] (73) via 192.168.102.34, dummy2, weight 1, 00:00:14

sharpd@eva ~/frr5 (master)> sudo ip link set dummy2 down ; sudo ip link set dummy2 up

D>* 10.0.0.0/32 [150/0] (73) via 192.168.102.34, dummy2, weight 1, 00:00:21

Ticket :#4198198 #4217209 #4224065

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

zebra/zebra_nhg.c

@@ -4007,7 +4007,14 @@ void zebra_interface_nhg_reinstall(struct interface *ifp)
 
 	frrtrace(1, frr_zebra, zebra_interface_nhg_reinstall, ifp);
 	frr_each (nhg_connected_tree, &zif->nhg_dependents, rb_node_dep) {
+		/*
+		 * The nexthop associated with this was set as !ACTIVE
+		 * so we need to turn it back to active when we get to
+		 * this point again
+		 */
+		SET_FLAG(rb_node_dep->nhe->nhg.nexthop->flags, NEXTHOP_FLAG_ACTIVE);
 		nh = rb_node_dep->nhe->nhg.nexthop;
+
 		if (zebra_nhg_set_valid_if_active(rb_node_dep->nhe)) {
 			if (IS_ZEBRA_DEBUG_NHG_DETAIL)
 				zlog_debug(