Francois reported that setting big mtu on loopback device could prevent
tcp sessions making progress.

We do not support (yet ?) IPv6 Jumbograms and cook corrupted packets.

We must limit the IPv6 MTU to (65535 + 40) bytes in theory.

Tested:

ifconfig lo mtu 70000
netperf -H ::1

Before patch : Throughput :   0.05 Mbits

After patch : Throughput : 35484 Mbits
Reported-by: NFrancois WELLENREITER <f.wellenreiter@gmail.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>