neigh: recompute reachabletime before returning from neigh_periodic_work()

If the neigh table's entries is less than gc_thresh1, the function
will return directly, and the reachabletime will not be recompute,
so the reachabletime can be guessed.
Signed-off-by: NDuan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -766,9 +766,6 @@ static void neigh_periodic_work(struct work_struct *work)
 	nht = rcu_dereference_protected(tbl->nht,
 					lockdep_is_held(&tbl->lock));
 
-	if (atomic_read(&tbl->entries) < tbl->gc_thresh1)
-		goto out;
-
 	/*
 	 *	periodically recompute ReachableTime from random function
 	 */
@@ -781,6 +778,9 @@ static void neigh_periodic_work(struct work_struct *work)
 				neigh_rand_reach_time(NEIGH_VAR(p, BASE_REACHABLE_TIME));
 	}
 
+	if (atomic_read(&tbl->entries) < tbl->gc_thresh1)
+		goto out;
+
 	for (i = 0 ; i < (1 << nht->hash_shift); i++) {
 		np = &nht->hash_buckets[i];