net/neighbour: cancel_delayed_work() + flush_scheduled_work() -> cancel_delayed_work_sync()

flush_scheduled_work() is going away.  Prepare for it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -1588,8 +1588,7 @@ int neigh_table_clear(struct neigh_table *tbl)
 	struct neigh_table **tp;
 
 	/* It is not clean... Fix it to unload IPv6 module safely */
-	cancel_delayed_work(&tbl->gc_work);
-	flush_scheduled_work();
+	cancel_delayed_work_sync(&tbl->gc_work);
 	del_timer_sync(&tbl->proxy_timer);
 	pneigh_queue_purge(&tbl->proxy_queue);
 	neigh_ifdown(tbl, NULL);