net: gain ipv4 mtu when mtu is not locked

When mtu is locked, we should not obtain ipv4 mtu as we return immediately
in this case and leave acquired ipv4 mtu unused.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1013,13 +1013,14 @@ out:	kfree_skb(skb);
 static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 {
 	struct dst_entry *dst = &rt->dst;
-	u32 old_mtu = ipv4_mtu(dst);
 	struct fib_result res;
 	bool lock = false;
+	u32 old_mtu;
 
 	if (ip_mtu_locked(dst))
 		return;
 
+	old_mtu = ipv4_mtu(dst);
 	if (old_mtu < mtu)
 		return;