zebra: add nexthop counter to 'show zebra dplane' command

The nexthop updates counter value was never displayed.
Add it.

> # show zebra dplane
> Zebra dataplane:
> Route updates:            7673010
> Route update errors:      0
> Nexthop updates:          1100
> Nexthop update errors:    0
> [..]

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

Author: pguibert6WIND

zebra/zebra_dplane.c

@@ -5984,6 +5984,14 @@ int dplane_show_helper(struct vty *vty, bool detailed)
 	vty_out(vty, "Zebra dataplane:\nRoute updates:            %"PRIu64"\n",
 		incoming);
 	vty_out(vty, "Route update errors:      %"PRIu64"\n", errs);
+
+	incoming = atomic_load_explicit(&zdplane_info.dg_nexthops_in,
+					memory_order_relaxed);
+	errs = atomic_load_explicit(&zdplane_info.dg_nexthop_errors,
+				    memory_order_relaxed);
+	vty_out(vty, "Nexthop updates:          %" PRIu64 "\n", incoming);
+	vty_out(vty, "Nexthop update errors:    %" PRIu64 "\n", errs);
+
 	vty_out(vty, "Other errors       :      %"PRIu64"\n", other_errs);
 	vty_out(vty, "Route update queue limit: %"PRIu64"\n", limit);
 	vty_out(vty, "Route update queue depth: %"PRIu64"\n", queued);