ipv4: fix ip_rt_update_pmtu()

commit 2c8cec5c10 (Cache learned PMTU information in inetpeer) added
an extra inet_putpeer() call in ip_rt_update_pmtu().

This results in various problems, since we can free one inetpeer, while
it is still in use.

Ref: http://www.spinics.net/lists/netdev/msg159121.html

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1593,8 +1593,6 @@ static void ip_rt_update_pmtu(struct dst_entry *dst, u32 mtu)
 			rt->rt_peer_genid = rt_peer_genid();
 		}
 		check_peer_pmtu(dst, peer);
-
-		inet_putpeer(peer);
 	}
 }