ipv4: avoid useless call of the function check_peer_pmtu

In func ipv4_dst_check,check_peer_pmtu should be called only when peer is updated.
So,if the peer is not updated in ip_rt_frag_needed,we can not inc __rt_peer_genid.
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1593,11 +1593,10 @@ unsigned short ip_rt_frag_needed(struct net *net, const struct iphdr *iph,
 			est_mtu = mtu;
 			peer->pmtu_learned = mtu;
 			peer->pmtu_expires = pmtu_expires;
+			atomic_inc(&__rt_peer_genid);
 		}
 
 		inet_putpeer(peer);
-
-		atomic_inc(&__rt_peer_genid);
 	}
 	return est_mtu ? : new_mtu;
 }